Funny thing about the first Amendment.
People have a right to say what they will (not unlimited, but there, nonetheless).
People have a Right to be wrong, factually, morally, logically, or all of the above.
People also have a Right to listen or not, embrace or reject the ideas contained in that speech.
There is no denying millions were slaughtered with industrial efficiency during WWII, during Stalin's regime, under Mao, and a host of other genocidal maniacs who inspired, incited, and led some of the most profound atrocities we know of.
If Fuentes wants to ignore that, the voices that rise postmortem from mass graves, from riverbeds full of bones, from ashes that rival volcanic eruptions, he is free to be wrong. Tucker is free, as well, to give him that platform from which to be wrong, and even elicit his enthusiasm for his error.
I have not seen the interview, but from past interviews Tucker has conducted with controversial figures, he lets them run where they will, and follows up rather than argues with them, so I'm not so fast to condemn him. That's just his M.O.