Actually, it's all in your definitions
This has cropped up before, though I've been looking at so many various sites I don't recall which one mentioned it, so I can't provide a link.
Apparently, 'external' power, in this context, is 'power not coming from the reactor itself'. So, both the *off-site* feeds from the power grid, and the *ON-site* power from the diesel generators are both considered 'external' power.
Don't know if it's true or not, but if it is, it still isn't as confusing to the average reader as radioactivity versus radiation.