Thanks for confirming! I updated my INSTALLATION.md to add that to the config.txt example as well.
Funnily enough, the RaspberryPi documentation claims that this flag
Defaults to 1 on Raspberry Pi 4, 400 and Compute Module 4, 4S platforms. Defaults to 0 on all other platforms. However, if the name given in an explicit
kerneloption matches one of the known kernels thenarm_64bitwill be set accordingly.
So I have no clue how this didn’t default to 1 when booting my kernel build. I bet they wrote a match based on the kernel name and just reset everything back to 0 when it’s not something already known.