Someone asked Andreas on that Twitter thread - it was the merging of the CSS Typed Object Model API spec.

This PR added ~6400 passes for some css stuff. That's not enough to warrant the spike I think, but surely helped: https://github.com/LadybirdBrowser/ladybird/pull/6370

You can't tell if it's a big jump because the graph doesn't show the axes. Maybe it went from 89% to 90.2%. The jump may also be no more than typical previous increases not shown off the left-hand edge.