Yes you're right he's leveraging a known brand. but marketing and branding - that _is_ business.
The point about staying in one industry is not about your technical knowledge, it's about your relationships. These are hard to build, and are the major barrier for all new ventures in any business: building your customer base, your sales channels, reaching out. That's tough.
The very fact that people know the name "Richard Branson" and that he's personally involved in all these ventures is a big part of it.
Edit: It's not a criticism--it's hard to succeed in different fields--but the basic idea of the OP, which is to expand your business ventures along the lines of what you know how to do, still applies to Branson a lot more than he's a counterexample of it.