I like how he writes, “even if the pizza has a reliable hechsher certification.” Why is that an “even”? If it doesn’t have it, you shouldn’t be eating it any day of the week!
When I was in Darchei Torah, the yeshiva used to order pizza for the melava malka each week. They also usually had leftovers from Shabbos. So they’d use one side of the dining room for pizza, and on the other side, they had the fleishig stuff. Some people would have pizza, wait a bit, then go have some cholent and kishka.