If I have an entire argument and my first premise for that argument is 2+2=5, then why would he let me continue when right at that point the argument is flawed?
Your choice of "2+2=5" is kind of a bad example because you're using a highly technical and accurate language with a single, unambiguous meaning. In normal conversation people use many words that have multiple (sometimes opposite) legitimate meanings and frequently even misuse words. They also misspeak.
Furthermore, it is a mark of Stef's experience debating and thinking rationally that he's able to immediately say, "That's not what I said," or "That's not what I meant," when people mischaracterize his position. However, it is an odd element of human psychology that people will often defend and argue for a claim that they don't agree with just because someone expresses an expectation that they do so. Often they will leave the debate feeling confused and as if their position was never addressed, because they ended up defending a position that wasn't what they meant. (As for why this is the case, I think compulsory schooling seems like a good suspect.)
If someone says something like, "2+2=5" I would suggest that Stef figure out if that's what they really mean* and whether it is actually relevant to their argument, since people frequently make inaccurate (by which I mean, not a clear expression of what they mean) and irrelevant statements, even when arguing. If it is not an accurate representation of their claims or it is not relevant to their actual reasoning, it's just a red herring, and listeners can go away feeling like their position wasn't actually addressed.
*Stef already does this a little, but I am suggesting that he should take it further. When people are talking with Stef, they often make almost incoherent claims. Stef will often work with them until they make a claim that is coherent enough to be wrong, but I am suggesting that he go further, as explained above.