They do have to confirm an S2 first.
It was #1 in several countries not just for weeks, but for the specific time window Netflix uses for evaluation. (It's slightly odd: for a subscription service and alleged breaker of conventions, they take a lot from the world of regular telly.)
Mary Sue spotted that a WGA credit for S2 has been put up, minus one writer from the original team (Lauren Bello - which would be a touch gutting, as she wrote the script for Sound Of Her Wings.)
HOWEVER: a lot of production houses will submit placeholder registrations if they think a sequel is inbound. And as we know from recent events chez WB, even having film in a can ain't no guarantee that a new series will happen.
As @neilhimself noted - he owns exactly none of the rights to the characters (and neither do Netflix if it came to a fight). DC do. But Neil owns the character's souls. It's a very peculiar vision and would need at least some of his blessing, which previous scripts didn't.
Showrunning Good Omens - which came out not just brilliantly, but on time and within a reasonable shouting distance of the original budget - also made Neil a bankable producer with a head for business, cinematic vision and entertaining audiences. So it makes sense to have Neil in charge.
That said. Netflix and WB are both in financial trouble, WB-D worse. HBO Max badly needs more hits to survive the reboot. The books have been flying off the shelves and merch is coming.And David Zaslav is a hardnosed shark, not a creative. If Zaslav smells money in the water, and S2 isn't yet at least optioned, it might not be the easy layup you might think.
Also, they have to pick which stories to tackle. The threats from Lucifer & Desire, plus Azazel's brief guest shot (oddly the only bit of CGI that didn't work so well)... they aren't just telegraphing. It's Neil jumping up and down with signal flags screaming Season Of Mists. But if they follow the path of S1, that's one half-season. They've already blitzed Preludes and Nocturnes and Doll's House, plus a couple of bits of Dream Country.
I would - and I doubt Neil is a visitor here, but here's hoping - politely ask for a couple of curveballs...