Disclaimer- I am not a Japanese speaker.
Don't worry about it.
The suffix -jutsu means 'practice of', the suffix -do means 'study of'. I have been taught by Japanese instructors who have used both or changed for emphasis.
Historically, martial arts tended to be called something-jutsu because they taught you how to do something....
The difference between iaido and iaijutsu is analogous to the difference between judo and jujutsu:
Iaidō derives from older Japanese martial traditions and has historically been referred to as “iaijutsu.” While “iaijutsu” continues to be used today, the term now refers to more traditional practices rooted in martial applications, while “iaidō” refers to ...