Yeah, that's what I would guess as well. But there's a big difference between "follow the [ridiculously complicated] spec to the letter" and "don't do obviously stupid things like filter out email addresses with short names". The latter is good advice, the former not so much IMHO.
For a couple glorious years I had a 2-letter email address at a single-letter .com domain. It was rejected a surprisingly small number of times.
Ah, that's a good assumption. My initial assumption was some sites have a very dumb whitelist of valid email domains. This seems more reasonable (although, also dumb).