Can you expand on this? Also, can somebody else provide a counterpoint?

Here's a good example. Someone I know is an AWS consultant. And, within that, he's fairly focused on billing/costs. He thinks it's helped him to be well-known for a specific set of things rather than be someone called in to consult about "public clouds" generally.

The counterpoint is that it needs to be a big enough market and one that you can service. Another thing to be careful of is that markets move on. I've known consultants who were world experts in some company's products or specific technologies. And then that company or technology ceased to exist and they really weren't sure what to do with themselves.

Not the author but I've experienced this. Basically what ends up happening if you don't specialize is you hit the 80/20 principle. 20% of your clients take 80% of your time. It becomes hard to scale and you end up not doing anything well. Focusing on one thing you can really focus on improvements to how you deliver that one service which reduces stress and helps you scale your service.