This means that in addition to any PSR-7 implementation, you can run apps using HttpFoundation (Symfony, Laravel, Drupal, etc) and "legacy" code using superglobals without the need for an adapter.

In that case I think I'm missing the value prop here, doesn't that also hold for plain old php-fpm?

That's not the case with other engines able to keep the booted app in memory to handle several requests (FPM doesn't support this).

Ah, it's a value-add compared to other engines that keep the app in memory, got it!