In the Pilot episode, Jeff, attempting to get test answers, debates with Duncan over whether or not truth is relative-
Duncan- "I'm asking you if you know the difference between right and wrong."
Jeff: "I discovered at a very early age that if I talked long enough, I could make anything right or wrong. So either I'm God or truth is relative, and in either case- booyah!"
Duncan: "Oh! Interesting! It's just that the average person has a much harder time saying 'Booyah" to moral relativism"
By the time Intro to Finality rolls around, Jeff has come full circle from this line of thinking, having dropped the belief that he can make people believe what is right or wrong by talking long enough. For reference, here's that final Winger speech.
Jeff: “Guys like me, they’ll tell you there’s no right or wrong, there’s no
real truths. And as long as we believe that, guys like me can never
lose. Because the truth is, I’m lying when I say there is no truth. The
truth is, the pathetically, stupidly obvious inconvenient truth is,
helping only ourselves is bad and helping each other is good. It’s that
easy [. . .] You just stop thinking about what’s good for you and start
thinking about what’s good for someone else. And with one move, you can
change the whole game.”
And so, Jeff accepts that right and wrong are not subjective. It's good to help people. It's bad to only help yourself. As it turns out, Jeff CAN say "booyah" to moral relativism, just not in the way that he thought he could in the Pilot. He knows what's right and what's wrong and he's done pretending that he doesn't.
And as if to underline that idea, minutes later, Pierce says "Booyah! Good person!" after admonishing Alan for using the word "gay" in a derogative manner.