The mystery deepens: ran the
2021-004, then, after booting back to HSierra, discovered that SIP was once agin disabled. Aha, so think to myself culprit has to be the SIP Disabler Patch, which works in one direction only from patcher to HSierra. To test, re-enabled SIP at the HSierra, from recovery, then booted to the patcher and ran
only the SIP Disabler, cache rebuild, booted to the Mojave, then booted back once again to HSierra and ran csrutil status.
Where, fully expecting to be seeing that csrutil status at HSierra was once again showing disabled, saw that SIP had
remained enabled.
Only other explanation I can possibly come up with is that something to do with cloning the Mojave HFS volume on the internal, now at 18G9216, to the Mojave APFS volume on the external, and then booting to that volume was responsible.
Otherwise, this remains a total WTF.