^It's actually been quite common for many, many years now to have 13-episode seasons on cable shows and in the UK, and it's certainly been known to happen with network shows too; for instance, both of Dollhouse
's seasons were 13 episodes long, though that's because neither one got the back-nine pickup. All four seasons of Blake's 7
were 13 episodes long, and of course, so is every modern season of Doctor Who
So yes, 13 episodes does count as a season, and has for a very long time.