I'm not an expert enough on the math to decide which study is accurate, the original or the rebuttal (nor did I read the original because it's behind a paywall). Nor, I doubt, is Trump. It's not an official government position that millions of illegal immigrants voted, it's Trump's personal opinion. Much like it was Obama's opinion that Trump would never be President (https://www.youtube.com/watch?v=-FkIJEmOyoA). Or that pulling out of Iraq and leaving a power vacuum wouldn't lead to the horrifying rise of ISIS and genocide (https://www.nytimes.com/2016/06/17/world/middleeast/isis-gen...). I think Trump's opinion is on the more banal side of things and won't have any severe consequences if he's wrong. That's just my opinion.
There's also a difference between being wrong about opinions (like statements about the future) and being wrong about things that are factual while making a claim that presents itself as a factual assertion. While the nature of what he's wrong about is banal, it represents an extremely dangerous disregard for facts. That he would make up facts out of whole cloth (or fully believe such a poorly done study because it suits his ego) should be extremely worrying to everyone. You knock Obama for his miscalculations, but if Trump's belief formation process is so defective when it comes to the banal, false beliefs are guaranteed to plague his decision making when it really counts.
[On the one hand, maybe Trump's false claim of illegal voters is wrong. But on the other hand, maybe he's right that ISIS and genocide are Obama's fault. So does it really matter if Trump is wrong?]
It's not an official government position that millions of illegal immigrants voted, it's Trump's personal opinion. Much like it was Obama's opinion that Trump would never be President.
Do you see a difference between someone having...
(1) an "opinion" about something that happened in the recent, knowable past -- but which flies in the face of all available evidence; and which, when challenged, you're unable to offer any evidence for -- like for example claiming that "The Kansas City Royals won the World Series last year", or "The Russians were the first to land on the moon", when everyone knows they didn't); and
(2) an "opinion" (i.e. a hunch) about what might happen in some future event -- who might win a football game, or an election, say?
