Given that they provide access to file storage via Webdav
https://www.fastmail.help/hc/en-us/a...l-file-storage
it does seem odd that one cannot similarly manage one's Notes using the same protocol. Seems to me it would be nice if Notes were really just text files in one's file space, under the hood, with the Notes section of the web interface being merely a friendly UI for accessing them.
I wouldn't be surprised to learn that FM also thinks it would be better implemented this way, but they have the legacy implementation in place, and it's just too much trouble (i.e. cost) to change it at this point.