This is not resolved and may be worse than I thought. The exact same condition appears when trying to use internal graphics with
M9 Apex BIOS 701, 801 and 906
M8 Extreme BIOS 1402, 3201 and 3401
three different 7530K - Kaby Lake BIOS versions
two different 6700K - All the above BIOS versions
In all those cases, the System Agent PCIe tab lists video BIOS as N/A and post does not find any VGA when there is no PCIe card installed. The white VGA LED remains lit while booting proceeds to its conclusion (Windows with the boot LED off or what should say 'press F1' for safe boot or a new processor with the boot LED on.) The same motherboards, BIOS, CPUs, cable and monitor work properly with a PCIe VGA. I think I'm doing something consistently wrong, or is there a consistent problem with all those versions of BIOS?
All those CPUs, cable, monitor etc work with internal graphics on another non-ASUS Z170 motherboard. I'll have to use that for the contest.