Companies at this stage--startups hitting stride--don't need "experimental R&D dev arms." The need laser focus on their main product, no distractions.
I think it's completely plausible that he doesn't get enough time programming and he's leaving because of it. Simple is growing. At this stage, he's a manager, not a programmer. Sounds like he isn't too interested in managing.
Too awkward? He wrote explicitly about why he was leaving: Simple has challenging problems, but they are not the challenging problems that light Alex up. Alex is interested in programming languages and dev tools, not tracking and routing money.
I don't have to speculate about that or wonder if there's "awkwardness" behind it. Alex said so. Just read the post.
You wonder whether people write posts like this because they anticipate speculation about why they're leaving or how the company is doing. Of course, there's no win to be had in being honest and direct, because people are just going to invent their own stories. The narrative that eventually sticks is the one that is most fun to bounce around, or has the most resonance to people, or is most congenial to people's ideology.
I think it's completely plausible that he doesn't get enough time programming and he's leaving because of it. Simple is growing. At this stage, he's a manager, not a programmer. Sounds like he isn't too interested in managing.