I believe that's consistent with the author's "coaster" description. "They work enough to get the job done" which is sometimes very little, but can sometimes

I believe that's consistent with the author's "coaster" description. "They work enough to get the job done" which is sometimes very little, but can sometimes be a lot if that's what's necessary. They're also a good source of slack -- "They’re also good for teams that have a lot of last-minute requests or questions". Coasters can do more work when that's what's needed. OTOH, a grinder always working at 100% can never give more.