Any reason why China wouldn’t be able to levy an extra 50% on the US?

Didn’t think so.

This is really unwise of Trump.

For whatever reason, Trump is taking the stance that China must immediately accept these tariffs without retaliation. But politically, Xi cannot be seen as submitting to Trump's pressure. So either China gives, or Trump gives, or neither gives and China is seen as openly defying Trump. Only one of those is a "win" from Trump's perspective, and it's the one that will never happen.