My guess is that the "appropriate selection course" for 21/23 at the time was Selection.
They have to do P-Company after selection. It is slightly abridged and does not involve milling (a bit too beastly for SF). Same goes for 22 SAS who are not already Parachute trained.
1 - to have completed the British Military Basic Parachute Course.
2 - serve in a 'parachutist' annotated post on an British Military Unit.
Nowhere does is say in QRs that you must have completed pre-para/P Coy.
How comes RN SPAG only get a lightbulb then? They are serving in an annotated parachutist post in a British Military unit.