[1]: https://en.wikipedia.org/wiki/Category:Songs_written_by_Mich...
[2]: I'm not trying to say that being a vocalist is somehow "worse" than being a songwriter, just that removing a skill he clearly had (writing songs) is weird.
Most of the songs I went through from Jackson where he appear to be a writer, as the history of him writing and producing the song. Of course, he got plenty of help. But I don't think it's fair to reduce him as using ghost-writers/producers for his songs, people seem to be properly credited.
I mean, it certainly takes a MJ to get to this level without music theory, but if the fact is confirmed... wow! (Tried to phrase it properly but "wow" conveys best that I'm speechless)
Reading sheet music is not music theory
If Michael Jackson indeed never learned to read music, then it would seem he is rather representative of the historical case rather than an exception. He grew up in a musical family and so he was always in an environment that constantly exercised the faculties involved. And we're talking popular music, after all, not some complicated orchestral work or composition.