Interesting, and likely true.
It's clear that FDR and Truman had significantly different world views.
Breitman's argument is not that, in perfect hindsight, FDR did everything he might have, only that, based on information available at the time, he did more than usually given credit for, and really, all that he lawfully could.
I think it was within his authority to grant asylum to a lot of these, esp. the “Ship of Fools” Jews.