Rowhammer itself has been around a while, and is only 50% of this attack that has been posted.
The other bit is the newer idea (well, old idea, newer actual implementation); memory deduplication by your hypervisor leads to a very minor timing fingerprint when you write to a page of memory that had previously been deduplicated IE the same physical page was shared amongst multiple VM's because it was identical.. until you wrote to it and the OS/hardware had to then Copy-on-Write it out to your own dedicated copy; that has a higher latency than a memory page that is already available directly for writing to you.
The other bit is the newer idea (well, old idea, newer actual implementation); memory deduplication by your hypervisor leads to a very minor timing fingerprint when you write to a page of memory that had previously been deduplicated IE the same physical page was shared amongst multiple VM's because it was identical.. until you wrote to it and the OS/hardware had to then Copy-on-Write it out to your own dedicated copy; that has a higher latency than a memory page that is already available directly for writing to you.