Technically, as noted, it's conditioned on SU upgrade class bucket availability. While they have typically not held back any C+ seats from the SU bucket, there's the possibility they could do so at some point (just as they hold back FC seats from OU Medallion comp upgrade bucket). Basically, they stopped simply providing "access" to C+ seats close to a decade ago when they decided to make C+ seats book into separate fares, instead of just being extra legroom seats that booked in Main cabin fares. Also, if there is more than one companion on the same reservation as you, no one is eligible for C+ upgrades since you can only upgrade one companion.