That's why the "???"

It's been a few years, but I think you're correct that APFS was optional at the outset.

I guess that if your installers ran, you're using the latest ones with current sigs, so I'm tapped out.

Maybe retry with APFS just to see if it runs successfully?

The new Great Equalizer is the SEND button.

In Memory of Harv: Those who can make you believe absurdities can make you commit atrocities. ~Voltaire