This could perhaps be the case but as tessus mentioned above, he doesn't use the spell checking feature. I have been a Joplin user for several years now and I have never used it either (as it appears not to even work on Linux) until I stumbled upon this issue.
Characterizing users with basic privacy concern as "ardent privacy enthusiasts" may be a bit hyperbolic.
After all, a lot of early users found out about Joplin from that original hacker news discussion. I suspect that many other users learn about Joplin from the highly regarded privacy tools and privacy guides communities or the dozens of articles on the web describing privacy respecting alternative apps.
The 'casual' you mention will typically be using whatever note taking application comes with Windows or may even search for an alternative and typically end up with the highly advertised Evernote or Google Keep.
A Joplin user has to go out of their way to learn about open source, community alternatives like Joplin and privacy is one of the key features that Joplin brings to the community.
Those other note taking applications have privacy policies that allow those corporations to use their data in ways that disempowers the user. Joplin on the other hand is a tool of user empowerment in that it gives the user autonomy over their data. Personally I don't want Google to be notified every time I write a note.
As the web and technology becomes more and more centralized, there is a real need for privacy respecting applications in the world. Joplin is a beautiful example of this and I think that is important.