The only definitively factual statement he makes is in the first tweet:
"One of the key questions about the new variant (B.1.1.7) is whether there is conclusive evidence that it is more transmissible. I don't think we are absolutely certain yet"
Everything else is speculation. He says that founder effect is not likely because the virus is spreading widely, nationwide: this is a hypothesis, not a statement of fact. He says that there are reasons to believe that the mutations are associated with structural regions known to be important to transmission: this is a hypothesis, not a statement of fact.
He then says that because of these two unproven hypotheses (which he implicitly believes), one should "change your priors" on how likely it is that this strain is selectively advantaged. Certainly, one can "change their priors" based on personal opinion, but that doesn't mean that other informed scientists don't have different opinions.
What this guy is doing is citing the (limited, non-definitive) evidence that we know, and then saying he has an opinion about what it all means. That doesn't make it factual.
Without well-controlled cell-culture or animal studies that show that this strain is out-competing other strains in vivo, we don't have solid evidence either way. Trying to predict the organism-level impact of point mutations is a fun parlor game, but no more or less definitive than asking a sports fan which team is going to win on Sunday.
