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.
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.