This was just based off the observation that, as of DC Universe Series 6, Mattel has now released four of the five major players in the "Reign of the Supermen" story from the early '90s. Why no love for Superboy? Well, who would want a kid hanging around when you're going out with your buddies? Not to mention his idea of "cool" was already about five years out of date by the time he was introduced.
The Supermen could have been doing any activity - the only thing that mattered was the exclusion of Conner. It's a pizza party because that seemed the least likely activity. Thus, comedy gold!
And yes, that's Steel talking like Hermes Conrad.
Superman: So we're all in agreement, then? Nobody invites the kid to our pizza party.
Steel: Daylight come!
Superboy: Hey, guys, what's up?
Steel: Uh, nothing, nothing. We were just about to, uh...
Superman: ...to... beat up Cyborg, here.
Cyborg: Wait, what?