The original post makes no mention of how a UI "done well" is different from an API "done well." I'm really interested in the omission and think that might be the only flaw in the argument.

I've never developed an API but the difference between 10 and 1,000 features is huge for the consumer of an API. With big enough APIs, even if the individual calls are beautifully simple and well-documented, I always wonder if I'm ever using the right parts just because there's so much there.