I’ve no idea what Swartz said on the subject and I’m not sure if this implication is intended by you but I would not characterize rms as just being uninformed. There’s lots of evidence in this piece that he encountered information and testimonials from abused parties and nevertheless landed at the conclusions he did. I don’t know if he holds CSAM or not but I do know he is long past the benefit of the doubt when it comes to his words.
Wayland has security benefits over X11 so choosing one with a Wayland session is a good idea but other than that I’m not really aware of any breakdowns of security between DEs. My gut says the major options are pretty similar on that front, most hardening guides only mention DEs to tell you which ones support Wayland. The choices you can make to harden your system are changing things at a lower level that is entirely unrelated to your DE. That said I’m just a privacy/security interested layperson and security is a topic where you can always split more hairs if you want (or need) to.