It's a rich text editor by default. Rich text is still text.
Opening HTML files and converting them to rich text certainly does belong as a valid feature for a rich text editor. It'll open and convert Word files too, which is super useful.
The content-type autodetection, however, I agree was a bad idea. Still, this vulnerability presumably existed with an .html file opened in TextEdit.
I assume the content-type autodetection exists because of how downloading files occasionally appends a .txt extension (I think this is when the content type is text/plain). Postel’s law gets applied with the result of macOS attempting to make up for misconfigured servers.
Opening HTML files and converting them to rich text certainly does belong as a valid feature for a rich text editor. It'll open and convert Word files too, which is super useful.
The content-type autodetection, however, I agree was a bad idea. Still, this vulnerability presumably existed with an .html file opened in TextEdit.