While I'm happy to run a startup as a single founder myself - I think I'd prefer to invest in teams too. A team already has something to match your investment (the team itself).

Also, I wonder if its not more difficult to judge which single founders will be successful, and easier to judge when it is a small team.

So I think treat this rule as investment advice, but if you're confident of running your own startup, trust yourself rather than PG's advice.