See https://doc.wikimedia.org/cover-extensions/AbuseFilter/, the footer says:
Generated by php-code-coverage 5.3.2 using PHP 7.2.31-1+0~20200514.41+debian9~1.gbpe2a56b+wmf1 with Xdebug 2.7.0 and PHPUnit 6.5.14 at Thu Dec 31 11:27:48 UTC 2020.
Which is very much outdated, and shows a very old version of the codebase. I eventually understood that it was generated by r652678, a commit against REL1_31 merged today. I believe these should not generate coverage reports, or at least not mixed up with master reports.