You’re right - the “SEM” reference was my mistake, and your question jogged my memory to go back and check what we actually installed ~3 years ago. Looking at our original migration-from-ADT shopping list, it’s not a SEM at all - it’s an IQ Hardwire PowerG (the hardwire-to-PowerG translator): Recommended Shopping List to Migrate from ADT Alarm/Pulse - #4 by jwcsurety
So to answer your question: the wired Back Door contact isn’t on the IQ4 directly (you’re right that it has no hardwired inputs) - it’s landed on the IQ Hardwire PowerG’s zone input, and the module presents it to the panel as a PowerG zone over RF.