Two things:
1. People leave companies unexpectedly all the time. For many reasons. People get fired, headhunted, win the lottery, die.. All valid reasons for leaving a company without notice. Without more info, it is not in and of itself enough to judge the company.
2. Interview wasn't agreed for 3, the start of the interview process was scheduled for 3 (a process that, for a 70K IT job could be expected to last quite a while). OP turns up early and finishes paperwork in 15 minutes, which is fine, but if you are early for something, you have to expect to wait. Now if OP had turned up at 3 as expected and they allow time for people to fill out paperwork and complete the test, say maybe 30 minutes to be safe, waiting for 15 minutes doesnt really sound that bad does it? Couple it with the fact that the secretary is there to make sure visitors are looked after, even if she is busy, it isnt unreasonable to assume the boss didnt think that waiting a little bit was as outrageous as some people do.
If anything, OP should have directed his snotty email to the secretary for making his first impression of the company a negative one. But that wouldnt fit in the fuck authority/champion of the common man delusion.