They don’t actually free the memory, but rather put it back into the memory pool with an incremented version counter. It can later be re-used as a new node, the version counter _not_ being reset.
In LeanStore’s use case they either way have a fixed amount of memory they’d like to use (since it’s a page buffer on top of disk data) so there’s no actual need to give the memory back to the OS.
If you’d like to actually free the memory back it’s a lot easier since you can now treat this more of an infrequent operation. The cost of that will be amortized very well.
EDIT: They also describe another very cool idea: If the memory is allocated with mmap you can deallocate it by using madvise DONT_NEED. This will cause the version counter to be “0” which you can interpret as “invalid”. You can then deallocate it at a later point when you’re absolutely sure no threads could access it. You’re basically doing a two-stage deallocation where the first stage releases the memory, but the page information is kept until the second stage.