You can edit the "products" represented in the table and add "Ladybird" to the list. [1]

Their result is: 1974740 / 2152733 (91%)

They also have their own dashboards tracking this [2]

[1] https://wpt.fyi/results/?product=ladybird

[2] https://grafana.app.ladybird.org/public-dashboards/2365098a1...

Here's a comparison including the big 3, ladybird, servo, and flow

https://wpt.fyi/results/?label=master&product=chrome&product...

To answer your question, yes. Apple requires 80% test passage of all the tests on web-platforms-test in order to be considered as a valid browser for iOS so they specifically targeted this suite to reach that milestone

It's a pretty silly requirement because wpt is not really meant to be representative of all web platform standards. It includes tests for non-standard features and the majority of tests are simple unicode glyph rendering tests.

I thought that no other browser engine could be provided on iOS. so no ladybird's engine, no servo, no gecko, no blink, only webkit

Some geographic regions have declared that not allowing other OS engines on iOS is anticompetitive so they're requiring Apple to allow them.

Apple is fighting it tooth and nail and coming up with requirements for other engines is a small way of doing that.

that was true until a few months ago due to a ruling in the EU. It's still currently the case that only WebKit can run on iOS but they're gearing up to change that

They are indeed just a subset of WPT. Although the way subtests are weighted in the score calcustion is slightly different for the "interop" score.