The way i understood this was that the gap in the argument was that: when the nuclear plants aren't upholding the safety requirement, they aren't operating safely. And AC B makes it clear that they weren't doing the safety stuff before the activists pressured them to, hence they weren't operating safely and when they had to operate safely, they had to shut down. So the activists were right that they couldn't operate safely.
The way i understood this was that the gap in the argument was that: when the nuclear plants aren't upholding the safety requirement, they aren't operating safely. And AC B makes it clear that they weren't doing the safety stuff before the activists pressured them to, hence they weren't operating safely and when they had to operate safely, they had to shut down. So the activists were right that they couldn't operate safely.