I think it's important to make a distinction between "someone, somewhere in the Judo community has used this at some point" and "this is widely known and used in mainstream or competition Judo". There have been Judo practitioners and training groups that focused heavily on groundwork - in particular, the Kosen Judo substyle. Just through convergent evolution, you can expect them to have come up with many of the same techniques and approaches that BJJ players use. That doesn't necessarily mean that one copied directly from the other, and it doesn't mean that you can expect to learn that stuff if you walk into your local Judo dojo.
That said, as far as I see, the approach to groundwork is different in the following aspects:
- Judo focuses on pins. That doesn't exist as a concept in BJJ - the concepts there are control and positional dominance. The back mount is seen as the pinnacle of control and dominance in BJJ, and it's worth squat under Judo rules, so techniques for taking the back and attacking from there are underdeveloped. Also, in BJJ, there's a greater urgency to keep looking for submissions when you have something that would just start the pin clock in a judo match.
- BJJ focuses on the guard a lot more than Judo, and has much greater depth when it comes to chaining submission attempts with sweeps, even from positions that would be considered purely defensive, like halfguard. Owing to the rules, "open guard" (one standing, one on his back) is not really a thing in Judo, and a huge field in BJJ. Funky positions like upside-down guard, 50/50 guard etc are likewise developments that arose from sports BJJ, as are the ways to deal with them (Berimbolos etc).
- BJJ has a ton of variations on choking techniques that are either frowned upon or outright banned in Judo. Guillotines in all their variations, triangle finishes that involve pulling the head, Peruvian neckties etc..