I suspect it's not just about optics--Xi is probably genuinely concerned about mRNA vaccines becoming another "chokehold" import that the US can leverage in anti-China containment strategy.

After what happened with semiconductors, China is never going to sole-source anything critical from a US-controlled or US-influenced supplier again as a matter of national security. E.g., Semiconductor export curbs hitting China to be followed by biotech and AI restrictions: US official

If/when China develops a domestic mRNA vaccine, that's when China will clear Pfizer/BioNTech for import, since both sides know if there's a domestic alternative, the US government can't use the foreign vaccine as leverage.