There is a big difference between saying R’ Falk is a talmud chacham and saying he’s a gadol.
His sefer on tznius has been widely criticized by many Rabbonim as being extremely (and unnecessarily) machmir. Does this mean he’s not a talmud chacham? Of course not! But it may mean he’s not a gadol. R’ Moshe Feinstein was asked what made him a gadol. He replied that people asked him questions, he answered them, and they kept coming back. When a Rav is labeled as too machmir and people don’t ask him any more questions because of it, then yes, it may mean he’s not a gadol. It’s nothing against him personally, it just means he doesn’t have that status.
I’m not belittling him, I’m just saying that most Rabbonim disagree with him. While he’s not a daas yachid, he certainly is in the minority. I’m sure the Rebbes from many chassidic groups approve of his standards, but most others don’t.