> To my surprise, the Swift one had the full Swift runtime embedded into it — about 5MB, while the Objective-C one was super light — tens or maybe 100KB in total. That's a huge difference, but I believe it's because Swift is meant to be somewhat cross-platform, right?
That experiment was done in 2015, when the Swift runtime had to be included. If you build an app now, it will link to the system runtime and be close to 100kb as well. This seems to be premature optimization. The author forced himself to learn archaic Objective C for a completely unnecessary reason, and now is stuck with that design choice despite not having any benefits.
It's easy, now, to say that it was a poor choice/premature optimization/unnecessary/whatever. But, presumably, the author is unable to tell the future. At the time when the decision was made, there were benefits that the author deemed necessary (smaller distributable among other benefits).