I think that’s wrong for a couple reasons. I think author doesn’t fully understand the problem or doesn’t explain it well leading to this assumption.

He wouldn’t care to get these extra jobs if he’s full, so why do this to begin with. He could however hire another mechanic if he books more jobs and grow his business to one of shop owner instead of mechanic (no idea if this is his motivation or not).

It’s likely he’s not actually under the hood all day but If phone rings twice a day and it just happens to be he’s under the hood at those times, he misses the call and it’s like he’s under the hood all day. It doesn’t mean he has no capacity, it just means he’s missing some calls throughout the day.

Perhaps it's a timing issue? Perhaps he would have time for more work but the calls cluster when he's busy?