My understanding is that it's the latter. "in SCED" basically means they have pre-planned availability that is cheap.
The "Physical Response Capacity" in that graph is the amount of capacity actually available, but it's not part of SCED. However it doesn't say anything about the timeframe it would be available in. Given that ERCOT didn't call for conservation, I would have to assume it was capacity that was "quickly available, but not cheap" rather than "not quickly available", but I don't know for sure.
Being in SCED just means that the resource bid into the real time energy market (which clears every 5 mins), it does not necessarily mean that the resource is cheap to dispatch. The confusion here might be caused by the differences of the ancillary market (PRC) and the energy market (SCED).
The graph showed it increasing fast just before. Is it so unthinkable it could jump again?
Or is that they could get more (non-SCED) in time, it would just cost a ton so it's avoided if at all possible?