We can’t assume anything president says is the truth
This is to propose that President Trump is not a liar. Liars sometimes speak or write untruthful statements knowingly and for self-serving purposes. Trump, on the other hand, so constantly and frequently speaks untruthfully that he should properly be called an untruther. One cannot assume he knowingly and for self-serving purposes speaks untruths. He just does, constantly.
Nothing Trump says or writes should anyone ever assume is representative of the examined truth. Liars are usually entitled to some benefit of doubt until proven otherwise. Untruthers do not have such an entitlement. Because of the constancy and frequency of their intentional or unintentional untruthing, nothing they say or write should be considered or assumed to be true until verified by objective evidence – no matter what their public or private office might be.
As a constant untruther, it would be a mistake for Trump to testify or speak with Robert Mueller since he would within five minutes speak untruths, aka as “lie to the FBI,” probably a felony.