Seems though as if the WPT score is not super meaningful in measuring actual usability. The growth of passed tests seems suspiciously uniform across browsers, so I guess it has more to do with new passing tests being added and less with failing tests that got fixed.
Fixing a few rendering issues could fix all of the tests that depend on correct rendering but break, so I think the rate at which tests are fixed makes a lot of sense.
https://wpt.fyi/results shows that even the big players have room for improvement, but also has a nice breakdown of all the different kinds of tests that make up the score.
They fixed ~10k tests, but indeed this month is a bit of an exception as there were lots of new tests added.