I can't remember if they ever refer to it as anything else in that movie but I actually appreciated this scene in the first movie for two reasons:

Info dumps irritate me in sci fi. He's like the main guy in charge talking to one of his lead scientists. They both absolutely fucking know why they're there. They know what it's called and what it's for. He's spelling it out for our benefit without breaking in-world character. If, in-world, someone started pedantically outlining what the rocks were for to their lead scientists, it would be the equivalent of calling them an idiot. Calling it "unobtainium" is like saying "we've had this argument before, I remember everything you said last time, you know everything I'm about to tell you, and nothing you or I do will change what's happening because you cant get it anywhere else and oh yeah it's worth a fuck load of money".

I can't remember if they later retcon that into being the actual name, but in that moment, it didn't sound like the actual name, it sounded like slang being used informally during a semi heated discussion.