It provides evidence in favour of him being correct. If there weren't other sources of information on Hubbard's activities, I'd expect him to be of genius-level intelligence.

Honestly, I wouldn't be surprised if he was; he clearly had an almost uniquely good understanding of what it takes to build a successful cult (though his early links with the OTO probably helped). New religious movements start all the time, and not one in a hundred reaches Scientology's level of success. You can be both a genius and a charlatan. It's easier to be the latter if you're the former, actually.

Although his writing's admittedly pretty terrible.

I'd also point out that if you read the investigative Hubbard biographies, you see many classic signs of con artistry: constant changes of location, careers, ideologies, bankruptcies or court cases in their wake, endless lies about their credentials, and so on. Most of these do not match Eliezer at all - the only similarities are flux in ideas and projects which don't always pan out (like Flare), but that could be said of an ordinary academic AI researcher as well. (Most academic software is used for some publications and abandoned to bitrot.)