HHVM was not a contribution to PHP. It resulted in PHP 7 being sped up and releasing with a bunch of long awaited features. But afaik , very little of HHVM made it back to PHP core.

It made PHP 5-7 usable in production, otherwise it would have died before we got to modern PHP.

Of course it wasn't merged in, it was a separate compiler, it certainly inspired future optimizations though.

But the point is, it was a very useful stop-gap solution for the community.

Also would like to highlight that they have contributed a lot to PHP upstream in addition to that.