As far as I know this is the popular defense against stick in Krav Maga. Are there any better alternatives for protecting yourself from stick attack. Is the defense in the video is relevant for defense from Tonfa as well?
You've shown a drill that demonstrates a theory. You should check out the Dog Brothers. They have a bunch of videos online. They have been very seriously training stick fighting for decades now. They go at full speed and power, sometimes without any protective equipment whatsoever. So if you see them doing it, it's probably safe to say it's effective and realistic.
I don't see them using this particular technique, but I could be wrong. They do have some stuff that's similar, but significantly different in many ways.
From my perspective, the technique shown in the video is unrealistic, because it relies on you rushing in quick enough not to get hit with a powerful Caveman strike. And you have to be quick enough that he doesn't see you coming and take a step back or step out of the way so that he can swing at you a different way, which is what he's instinctively going to do.
The other thing I noticed about the demonstration in the video is that the one who's attacking with the stick is going fairly slow and light. Yet, the defender must go at full speed in order to be successful. That should raise suspicion that maybe this won't work if an attacker is going at full speed and power.
Also, the angle of the swing used in the video is a bit too horizontal compared with a true Caveman strike. I would think it's more realistic for an attacker to use a more diagonal, almost straight up and down strike to club someone on the head. Such a strike would be more powerful than the one shown in the video, and more importantly, it would be harder to defend against.
And finally, the reason why most martial arts tell you not to attack his swinging arm directly (with both of your hands and with all of your focus) is that it's a very fast moving target that can be taken away very quickly. So for example, he might see you going for his arm, and all he has to do is pivot 90 degrees to get his stick away from you so you can't grab it or the arm that is controlling it. And at the moment he does that, you're now in a more vulnerable position with both of your hands now away from your center line, no longer protecting your head.
My instinct (and this is just me) in this situation as a defender would be to step in diagonally towards his left side. This is towards his weak and open side, in other words. This angle immediately gets me out of the way of his Caveman strike. He might still be able to tag me, but not as powerfully. I would keep my left arm high to defend my head and be able to grab his right arm if he tries to swing at me from this position. That leaves my right arm free to connect with him in some way. Connection will allow me to keep the distance so that he doesn't get a chance to do a powerful swing (it's still likely I'll be hit, but not as bad). I might then try to grab around his back with my right hand and try for an o-goshi nage, or I might go low for a single leg take-down grabbing his left leg. When he's unbalanced, he can't use that stick very effectively.
A few rounds of pretend stick fighting will tell you if this drill shown in the video works or not. I highly recommend it. Three minutes of this is worth more than 3 decades of theory and drills.
You can use padded sticks for this experiment. No need to go to the level that the Dog Brothers go to. You just have to tell your partner to actively try to hit you at full speed and power. And he's allowed to change up his technique however he wants. He can even kick or punch you if he wants. He just has to start by doing the preparation for a Caveman strike (which is what this is). If he sees you coming and is unable to do the Caveman strike, then he should either move to a position where he can complete the strike or change to another kind of strike and really try to nail you.
And to make it more interesting, don't tell him what you plan to do. Simply tell him his goal is to hit you and stop you from doing whatever you want to do. He should not try to move in a compliant, accommodating way. He should be resisting you fully and using his brain and his instincts.
What I'm getting at is this. You have a video showing a drill. It's a theory. It looks neat and effective. But you won't know if it's really effective until you try it in sparring or real life. And I suspect you're going to find that you can't make it work in those scenarios.
You asked if there was anything else to use in this situation to defend yourself from a Caveman strike. Yeah, of course there is. But that's a very open ended question. There are literally thousands of different things you can do. And all of those will just be theories as well.
Why don't you explore some of your own ideas in the sparring experiment I've outlined above? Think of other things you can do from this position. You're going to get a lot out of that I think. Let that be your teacher.
As for the use of the Tonfa (side handle baton) instead of the stick, there are lots of different swings and holds you can use. So again, the question is too open ended here. I will just say see if you can find the answers on your own through the sparring practice I outlined above.
Hope that helps.