I don't know the details, but I find the claim that a 32-bit program couldn't use dark mode hard to believe. But, even granting that, it's pretty unusual to have to rewrite much just to recompile a program in 64 bits. Most of the time, it does not take any changes at all.
It doesn't mean that at all. It is very rare for Windows updates to break reverse compatibility. A lot of 20-year-old software runs perfectly fine without changes.
It's really not rocket science. Saying this having written a full-featured Evernote client myself a while ago, one with rich text editing, offline note storage, search, attachment, multimedia note support, etc.