While that is technically correct, they haven't been running vanilla PHP for quite some time - it's jitted into x86 and run natively on the machine. It's also pretty extensively optimized. And the VM that does the jitting is all C++. Their MySQL deployment is also an internal fork that's been heavily modified for scalability (storage/compute separation, sharding, RocksDB based storage engine instead of InnoDB, Raft for synchronous replication, etc.). Lots of really fantastic engineering work has gone into making their backend infra scale.
Some worthwhile references:
[1] https://engineering.fb.com/2016/08/31/core-data/myrocks-a-sp...
[2] https://research.facebook.com/file/529018501538081/hhvm-jit-...
[3] https://research.facebook.com/file/700800348487709/HHVM_ICPE...