This is due to the PHP7.0 version of the Debian version installed by default on the Raspberry (Raspbian), which is still 32 bits. There's no way (to my knowledge) to get a 64 bits PHP version in a stable enough manner in Stretch. We have to wait for the Buster version of Raspbian, approximately mid 2019.
Anyway, this is not a problem (yet), since Moodle only drops a warning about this, because in a future version (but not the 3.7.x one), it should require it.
Edit: In fact, Raspbian is currently 32 bits only (even if RPi 3B and 3B+ are 64 bits chips). That's the reason why we don't get 64 bits versions of the programs.