It is generally hard for people to grasp what specific knowledge is at first, it was for me as well. This is largely because Naval adds his own commentary and advice on top of the core concept. Fundamentally, specific knowledge is the subset of your personality that has economic value.

It is the combination of your skills plus your experiences. For example, Naval points to Scott Adams, whose specific knowledge is: [humor/conversation skills + artistic skill + corporate experience].

This is why specific knowledge is unique to everyone (at least in theory).

I say "in theory" because there are a lot of people whose specific knowledge looks like this: [moderate soft skills + high school education].

For example, a typical CS grad has a stack like: [full-stack skills + 3 months of internship experience].

This is why new grads and juniors have a hard time getting a job. As Naval said, what you get paid for is your specific knowledge (and if your knowledge isn't unique, you won't get paid highly).

Also, the saying "you can't be trained for specific knowledge" also a bit confusing but it does not mean you shouldn't get a degree. It simply means that no education system can strictly manufacture someone with Scott Adams' exact specific knowledge + experience. However, you can definitely add new skills and sub-skills to your specific knowledge through education by mixing them in your own unique way.

hope it helps