LOL I opened this thread just to post a link to the Quanta magazine site. It's a very well done article and applies a great human element to the story. Dementia is one of the few things that scares me.
For an amateur mathematician to do this is, as you say, a surprising feat. I'd go beyond that to add a bunch more superlatives.
It is stories like her story that serve as good reminders that the idea of citizen scientists is healthy and should be encouraged.
I highly recommend the link you provided. It's a bit long but well written.
And another that I just found and read:
https://www.conquermaths.com/news/post/index/264/Marjorie-Ri...
Personally, I love this webpage partially because it's just so different than what I'm used to for the pages of mathematicians. It really comes across as made by someone who was extremely talented at math but who only saw it only as a pastime and never bothered to integrate into the mathematical community.