In fact (don't know full facts, but I too have been reading his blog for some time), I think he was a designer (of mobile apps and web sites, maybe had dev skills too), then wrote a few books for a while (on areas where he had skills, like design, later on other areas too), then used the earnings from that to start Convertkit. IIRC he even mentions this approach in one of his blog posts. Bootstrapping all the way, seems like, to me :)