Trying to figure out whether a B-748 flight with a Cathay callsign operated by Polar which is owned by Atlas using U.S. Op Specs is legal for an RF departure out of VHHH is kinda like trying to find a lost bag on a codeshare ticket.
Not quite...
The key is "operated by Atlas Air". Atlas crews operate under Atlas/Polar OpSpecs, which are essentially identical. Radius-to-fix SIDs are explicitly allowed.
OTOH, IIRC an RTF SID in HKG must be explicitly requested by the crew. If the crew is unfamiliar, they may not bother with the request.