Plus I would assume that it’s only permitted in a situation in which you feel that it’s really important to convince someone of this truth.
I think he says it in relevance to Limmud Hatorah as well – if you are arguing over a sugya and you are convinced of the validity of your claim (which can often happen without an explicit source), you any lie and pretend to have seen it somewhere else. I haven’t seen it for some time, I’ll bli neder look it up.