Isn’t that a misdiagnose? You are checking the ENV of a brand new spawned ruby proccess instead of the one running the actual web server.
If you inspect the /proc/<pid>/environ of the Pitchfork mold process you will see the YJIT env var there.
root@raspberrypi5:/var/discourse# cat /proc/3331660/environ | tr '\0' '\n' | grep -i yjit
RUBY_YJIT_ENABLE=1