For all those complaining about the Guardian retcon, it's pretty obvious the story Gendy went with for this final season is not the ending season he had initially planned when writing the show a decade ago. This could be for any number of reasons. Maybe the number of episodes they gave him (10) wouldn't work with the ending season he had planned which might have been based on having more episodes like the old season got. Maybe the venue switch to Adult Swim gave him the ability to have more freedom to the amount of violence and borderline nudity and it let him make an ending season he had originally wanted to make more but couldn't originally when the show was on main cartoon network. Maybe the script he wrote for the movie that had been teased was what he based this season off of instead of the original plan back a decade ago. Or maybe after a decade of hiatus, he just changed his mind and decided on something different.
Any one of these explanations could explain the change in plans. It might even be a few of them mixed together.
That being said, when marathoning the whole show now, it's probably best to just skip the guardian episode from here on out. It won't completely fix the issue, but it leaves it overall less co fusing if it's not there.