From the linked article,

A program known as convergence bidding, in particular, is meant to help keep electricity prices steady but instead “masked tight supply conditions” during the August heat wave, the analysis concluded.

and

Energy providers collectively under-scheduled the amount of electricity they expected to need.

This was definitely a lack of capacity.

As a European, it always baffled me that power outages are such a common phenomenon in the US. I've heard stories about rolling blackouts due to heat/cold from people in all areas in the US. Whereas in Europe, I cannot remember a single instance rolling blackouts were used because of a gap in supply.

Sure, Europeans pay more for electricity, but maybe that spare capacity and inter-connectivity is worth it.

On any given day this month I've had an 87% chance of having uninterrupted power. 27 hours of total downtime, despite paying some of the highest rates.