This Isocracy quote is maybe mixing memories from the show at Eggplant's in October 16, 1988, where John Kiffmeyer met the band, with this last Operation Ivy gig. Because this definitely was not Green Day's first performance.

This also brings me doubts about the other acts like Crimpshrine and Isocrary on this date

Alfie - The myth of "the first Green Day show was the last Op Ivy show" has been spread around for years (mostly attached to the show the day prior at Gilman) despite it not being true, so most likely they just believed it to be true based on that myth. The show the day prior was their first show at Gilman with their new name which is probably where that whole thing started. I've added an ed note to the footnotes to make it clear that it was not the first Green Day show.