An acronym is a kind of initialism that can be pronounced like a word e.g. NASA. Your reasoning for why it can't be pronounced as a word ("sequel") depends on it not being an acronym. But, since many people do pronounce it that way, it is, in fact, an acronym and can be pronounced that way.
Ess Queue Ell. I didn't hear the sequel pronunciation until I was well in my thirties and even then only by people older than me. Then I started working with it myself and heard people younger than me pronouncing it sequel, so now I have no idea. I still hear Ess Queue Ell from the vast majority though.