I'm not going to lie to them, I think that would be wrong and try and tell them it's actually Windows, though if I were upfront and just made the icons look like Windows that would be ok. Even with the default Cinnamon icon styling, if I put Chrome on the desktop they'd figure it out, but even if I were to somehow objectively prove that OnlyOffice or LibreOffice or Calligra or <insert Linux office application here> are better than Microsoft Office, unless I can get the desktop version of Microsoft Office working on there I'm pretty confident that they won't go for it.

I recently tried to get my mom to use the PWA of Word online, and she said it was "worse" than the desktop version but refused to elaborate as to why.

I know I sound like I'm being harsh on my parents, but it's not like they're stupid or anything. They have spent the last 30+ years using Windows and Microsoft Office, I don't blame them for relying on the tools that they have spent the last three decades learning; I use command line tools that have existed longer than I have [1]. I do understand the stubbornness even if it does drive me a little crazy.

[1] Yes I know that GCC and Emacs and tar and grep have gotten updates since they've been released, but so has Windows.