It's pretty interesting that there's so much variation in how we're understanding the advice.

I read this as John suggesting a stair step path to "learn deeply". If you can section off one defined area, a tool, it's possible to learn _that_ inside and out in a reasonable amount of time. Then you're useful and people want you around. From there you can expand outward or down or whatever.

This would be as an alternative to dabbling, learning the most commonly known things across a wide area.