He was told they weren't true. Nazis were the first to spread them.
He knew the Haitians in Springfield were legal immigrants (they have temporary protected status- a George Bush Sr. creation for refugees from countries in crisis).
As my friend said, the Springfield narrative should be especially concerning. Vance and Trump so easily attacked legal immigration status.
Recall that towards the end of Trump's first term, Trump's Justice Department created an office to denaturalize immigrants of their status.
Stephen Miller has promised to expand those efforts in a second Trump term.