On reflection, I can’t really buy into this. Maybe it’s because I am (or was before retirement) all three. I agree with the do one thing well, up to a point, but here’s the crunch.
Doing ONE thing requires supplemental doing other things to succeed with the one thing. So sorry, not buying what sounds good but is an overly simplified notion of things. I’m not a web designer, but have designed a lot of websites, in the service of what I “really’ do. But I do design, as a support piece for writing, for generating sales, for building business, etc.