I read that before posting. And it's talking about potentially doubting מרע"ה as the messenger from the רבש"ע. Not about doubting "the existence of Hashem" ח"ו!
That's an interesting distinction that didn't occur to me previously, however I'm skeptical if you're right. He's explaining ויאמינו בה׳ ובמשה עבדו, and that's talking about both of them. He also cites Yisro, and the Pasuk he cites clearly says Yisro was only convinced about the existence of Hashem by Krias Hayam, nothing about Moshe ( עתה ידעתי כי גדול ה׳ מכל אלוקים, כי בדבר אשר זדו עליהם).
We're talking about בני ישראל.
We're discussing whether or not the Makkos were
evidence of the existence of Hashem, not what anybody believed.