Km, no, it was the logical trajectory of his teachings that led them astray. If you don’t need to accept X in yiddishkeit, then why not this part, or that part, or…the whole thing? This is why the rambam says that one is who is kofer in one letter of the Torah is the same as being kofer in all of it.
Mitzvos aren’t all or nothing, but emunah is. Emunah is having complete subjugation to the words of torah shebichsav abd and chazal, and mendelssohn was faulty in both. Maybe not to the extent where he himself broke halacha in practice, but it was enough to lay the groundwork for a wholesale abandonment of Torah.
For the record, rav yaakov emden had a correspondence with mendelssohn, and said at the end that the whole time he was trying to be mekarev him, but is giving up because now he has strayed so far that he owns a dog – this was his sign that he was off the derech.
What always puzzled me is that among the gedolei yisroel, there was one who i read supported the german translation/interpretation of chumash that he wrote, and even wrote a haskama to it, and that’s rav akiva aiger, despite his son ij law being the chasam sofer, who was very outspoken against mendelssohn.
But in light of everything above, rav Akiva eiger is the kasha, not the terutz. Maybe he saw nothing wrong in the biur and wanted to be mekarev him…who knows.