Looks exactly like what some of us reported with the last security update. The only difference being, there we were able to boot into safe mode, re-install the update, and eventually get it to boot normally again.
I really don’t understand what’s up with Apple’s QA/QC these days. With their tens of thousands of employees and millions of developers, their betas, public betas, and all the other hoopla, how hard can it be to test these macOS updates on more than just one config of the latest MacBook? How can you advertise the longevity of your hardware, when the required security updates tend to brick that hardware once it’s no longer the latest and greatest?