After replying to an e-mail in the bugs@openbsd.org mailing list regarding the borked bootup, Theo de Raadt replied and confirmed that a snapshot went out without the commit for the ABI break, and it was replaced by a newer snapshot that did have it. He recommended updating using a miniroot snapshot.

Downloaded the latest miniroot78 snapshot from an #OpenBSD mirror, brewed some water for my mate as the miniroot78 snapshot image written to a USB drive, and then booted from it on the laptop. Update went through wonderfully and I'm back to a booting OpenBSD system. 🧉 ​​​ #RunBSD