People who do this type of thing are truly valuable to the community as they improve readability and make answers easier to understand for other users.
Yes, I completely agree with this. Note that if that was a user with less than 2000 reputation, they did this via a suggested edit and they already got 2 reputation for having the edit accepted (=marked helpful).
IMHO, the best way to pay it forward would be to significantly improve one of their posts. If they don't have one (that you can improve), pick another post on the site which needs some polishing. Or just contribute to the community as a whole by writing another good post.
Editors can be 'pinged' in a comment so you could use that for thanking them; however, that's not what comments are meant for and you should definitely remove the comment after they've read it.
I am my bot is making quite a lot of edits across the network nowadays. What I experience is that sometimes after such an edit, I get a random upvote (or, less often, a downvote) on one of my posts on the affected site. That is not how it should work: you should vote for the content, not for the user.