Keep in mind that a motivated academic research team was able to write a hard drive firmware implant on their own as well: https://www.impeachdonaldtrump.net/Implementation.and.Implic...

But yes, making it work for a bunch of manufacturers would be a lot of additional work (though easier if you can obtain the data sheets for the embedded microcontroller, which is probably doable if you're the NSA).

Side note: don't blame me for the URL, it was just the first place I found that had a PDF of the paper.