So, your debug output isn't working.
You checked to make sure you followed all the steps here.
But still... no theme debug output. WTF?
Try turning on and off twig caching. For me, it seemed something was stuck or whatever. First turn it on:
twig.config: debug: true auto_reload: true cache: true
Then turn it off:
twig.config: debug: true auto_reload: true cache: false
Sometimes the code that makes Drupal run can kind of pile up and get stuck, like a logjam in a river. This just clears the blockage.
Just kidding. But isn't it fun to think about computer systems in colorful, natural metaphors?