It’s taken me a while, but I finally tried opening 2 Settings sessions at once: Update was running already, and I tried opening Display Settings too. All that did was switch the one Settings window to Display Settings, with no back arrow in the top-left corner.
Looks like Settings is truly a UWP-style app, and it can only handle one instance. If so, for PCs it’s a definite step backwards from Control Panel.
I wonder how long it’ll take MSFT to permit multiple simultaneous instances of Settings sections.