I'd bet it's a combination of these things. I know a few people with moderately successful podcasts, and the core thing is this: getting prominent people on your podcast is easier than most people on the outside expect, and harder than most people seeking guests expect.

Prominent people generally like getting their ideas out there, and having a 30-120 minute conversation is a relatively scalable way to do that. Sure, they get asked by a lot of people, but that's where the other points come in.

Let's take prominent guest X. They might get 100 podcast interview requests a month. They might do 1-2 podcast appearances per month (average, most guests cluster their appearances for PR purposes). From the very start, Lex, through his MIT stature, first and second second-degree connections, etc. would very likely be in the top 2% of the 100 people asking; most people asking are some combination of no audience, not targeted in their ask, abrasive, etc. If you're in the top 2% of 100 requests, and they fulfill 2 requests per month - you're looking pretty good. Throw in an element of "luck" in the first few guests if you'd like.

That's just getting started. Once the flywheel starts, it's easy.