I am not sure what video you looked at but the one I saw shows clearly FDT answered the FairTax fellow “Yeah, absolutely!” to the question “if Congress passed the FairTax would you sign it into law?”. After FDT answered yes, the FairTax fellow said “Fantastic!”
That was unequivocal.
As to your assertion that Fred hadn’t heard of the FairTax until recently, I find that extremely hard to believe as one of his closest friends and advisors Congressman Wamp is one of the sponsors of the FairTax bill in Congress, HR 25.
From the Fair Tax website: “Mitt Romney...lost ground when he declined to take a position on whether a national sales tax should replace the income tax. (He said he hasn’t had time to study it. But he’s been saying that for months now...)”