He has gone in deep on performant low-level audio code, and how it integrates with the system APIs. He’s done a lot of interesting stuff with programmatic drawing of icons in his apps. He did a lot of good caching work back in the Instapaper days, when cell connections were almost like dialup.
He seemed to be reluctant to learn Swift, because Swift would have gotten him…what? I think he thought that Objective-C was mature, tested, comprehensible, and battle-tested in production. And it wasn’t going to change out from underneath him…which you sure couldn’t say about Swift for the first few years. You eventually had to adopt it, as Apple is moving to Swift-only, but I think Objective-C let him accomplish his goals, and a lot of the good security stuff in Swift is maybe not super relevant to his app development.
(Ugh, PHP, though…)
His use of Obj-C and PHP are just as pragmatic to me. He's an expert at Obj-C and possibly at PHP, and why change? Let Swift mature, see if Obj-C becomes deprecated, and then move on. He can obviously learn new languages since he's dabbled in Rust etc.
I do wish ATP focused more on tech stuff and less on how to live your best life on Fire Island. But I enjoy it every week.
My impression is that Casey is quite weak (as in average), but meticulous.
Siracusa is almost certainly the one with the best understanding of theory, but hard to say how he is practically. He could be very good at what he's doing.
Marco also doesn't seem very strong in raw programming (he resisted Swift for half a decade, complains that it's hard to deal with, says that architecture is only for beginners etc) but obviously he can solve whatever problem he is faced with, even quite complex ones. And this is obviously what matters if you are an indie developer. That and product sense, which he is also very good at. He probably has the perfect skillset for an indie developer, better programming wouldn't make him any more successful.