I think as they ramp up closer to the premiere date, they'll really start pushing the Superman angle; and they probably feel that just mentioning it's a Superman show will get an audience without needing a lead in from The Flash.Huh. Curiouser and curiouser.
As much as I would be up for a two-hour Superman & Lois feast that first night, I'm frankly not sure this is the best idea (if true). The Flash remains The CW's highest rated show, and S&L's premiere might well benefit from its lead-in. OTOH, maybe the execs got excited over the S&L footage they were seeing, and decided to give it a bigger, splashier standalone debut? We shall see.
I think they're going with this also because Superman and Lois is not going to be like the CW's other DC superhero shows, so perhaps they don't want to have a lead in from a superhero show that's very different from what they expect Superman and Lois to be.