I just did a websearch for my username + reddit. Found a comment I made just 10 days after I created the account, arpund 2 years ago, and it now shows the new text I instructed the script to edit all my comments with.

I can therefore higly recommend this fork of PowerDeleteSuite

https://github.com/pkolyvas/PowerDeleteSuite

As it seems no comments were left untouched! I checked old.reddit and it indeed stops much earlier (I think around 1000 comments like the other user said) which for me is just 9 months ago (so I may have not noticed this the first time and assumed the last page included the very first comments), but since I found a 2y old comment like I said above and it was edited correctly by the script, I can say it works lol