This is not true.
Silver's reading comprehension is terrible.
The "proximal origin" paper specifically included cell culturing in its possible theories of the origins. The only thing it specifically ruled out was an engineered bioweapon due to the genomic features of the virus. The paper in its conclusion said a lab leak origin was not plausible based on the analysis and reasoning presented in the paper, but it specifically didn't rule it out.
Kristian Andersen in his Slack messages said very much what he said in the previously released emails. He initially entertained a lab leak scenario, but after conferring with coronavirus experts he realized the aspects of the virus he thought were unusual were actually not unusual. The analysis that changed his mind is in the paper itself too.
I swear it is like these dudes don't actually read the paper they keep talking about.