mwscript has been hardcoded to use php5 it should use php and thus HHVM when host is set to default to it.
The reason we still use Zend is there was some code (unknown which) that was erroring out in hhvm during scap. The fix was to hardcode php5 back in February with https://gerrit.wikimedia.org/r/#/c/267816/
I don't know whether the issue is actually fixed in HHVM (or the mediawiki codebase?)
When creating a deployment server with Jessie (which has HHVM), scap fails to run ExtensionMessages-master.php because the Zend extensions for Memcached or Redis are no more installed (T146286).