My guess is they used # of PRs as a measure as it’s easy to obtain, while other measures are hard, may be due to other factors, etc.
FWIW I saw a similar number for myself, around 30% more PRs in the last 6 months, compared to the 6 months before that (I picked up agentic coding around at the start of the year). And a similar increase for closed issues.
In my case this clearly doesn’t translate to as much value for the organization, or rather, it’s hard to say, as many of those PRs were things I wouldn’t even have done without AI support. This means they were low priority. However, many were of the cleanup/refactor type, so they might result in speedups later.