The ATF will definitely swell some otherwise impervious plastics (like the Nylon in a Glock) and ruin them. I have a Marlin Camp 9 in the safe right now that I am having to replace the entire trigger group on because some fellow used Ed's Red (which contains both Acetone and ATF) on it.
I'd put my money on the acetone being the culprit.
ATF might be a bad idea for other reasons, but I have a hard time believing it will damage nylon. I can't find any support for the idea that nylon and ATF are a bad mix. To the contrary, I find several products which are in contact with ATF and use nylon.
Automatic transmission filter
A component in most automatic transmissions that is fitted over the fluid intake, usually in the pan, that prevents debris and other contaminants from circulating throughout the transmissions inner workings and potentially causing unnecessary wear or damage. Some transmissions use a reusable metal or nylon mesh-type screen in place of a disposable filter.
Both times the instructors from Glock warned about Automatic Transmission Fluid, particularly gumming up the small parts.
This was omitted from the course I took. I'll ask this week when I recertify. Either way, gumming up small parts is hugely different from ruining the frame.