Read it again. The vast majority of THEIR voters,

He's talking about Republicans, and I strongly suspect that analysis of districts represented by Republicans will show most, if not all, were carried by Trump. States with 2 Republican senators were also mostly carried by Trump. States and districts carried by Hillary are more likely to be represented by Democrats.

So yes, the assertion that Trump was supported by the vast majority of voters who also sent Republicans to Congress is likely a correct one.