What to do, half the house has no power after I used the GFCI tester
My understanding was that pressing the gfci test button when the device is plugged into a non-gfci would be a non-event, since very little current would actually flow. And yet, here we are.

Correlation is not causality.
Your understanding is correct, and the power loss was due to something else entirely.
My money's on &quot;It tripped a GFCI that you don't know about&quot;.
I bet it tripped the breaker because it's a GFCI breaker (those have Test buttons)...  And I have a feeling it's in a subpanel, which is fed from another GFCI breaker.  So now you need to hunt that down.