I'm sure that some illegal immigrants do find a way to vote, but it's hard to see this number being remotely close to "millions." There are about 10 million illegal immigrants of voting age; saying that 20% of them are somehow registered and voting seems absurdly high given the risks and restrictions. I could see maybe 1-2% at most going for it.
You'd have to believe that non-citizen voter turnout is at or near levels of
citizen Latino turnout (as a share of their total populations). Anybody who knows the first thing about how non-citizens behave with regard to government institutions would find this laughable.
Then, you'd have to have African-American levels of support among non-citizen voters for Clinton just for Trump to break even in the popular vote.