It's not clear to me whether this means physically or economically; i.e., could the 2.0 satellites be put into orbit by Falcon but not at a cost that would make it worth it?
It is always a mistake to assume his tweets mean anything specific. He means they won't be launching 2.0 birds on Falcon, full stop. Reasons are slippery things.
"Falcon neither has the volume nor the mass [to] orbit capability required for Starlink 2.0," Musk said.