It means it only has a PM indicator...the square box to the left of the "PM" would be an outline instead of filled when it's "AM". Similar to many old LED clocks that just had a single LED to indicate PM.

The stackexchange post has a link to a different implementation that has separate AM/PM indicators instead: https://codegolf.stackexchange.com/questions/88783/build-a-d...

Direct link to a runnable version with AM/PM: https://copy.sh/life/?gist=9d7468755dd76a35d93beeb5c0a5bdcf&...