I always first try and pull some slack through the clamp or connector if possible. If not, I will do exactly as you described.
In older homes where 60C NM cable is present, I always connect pigtails of 90C NM-B wire to comply with the light fixture listing that they be connected to min 90C wiring.
As far as 300.14 goes, it could probably be argued that there doesn't seem to be any wording that indicates that the required free conductor has to be one continuous piece and since this pigtail conductor doesn't originate from outside the box through a separate KO entry, it doesn't count as any additional box fill according to 314,16[B],1.
So, IMO these pigtailed wires would be considered a single conductor and the wirenuts also wouldn’t need to be counted in box fill either.
Seems like a win, win situation for once.