Not that me creating something that rhymed on the fly in less than a second should be defended from literal scrutiny....but the point of saying "I could sit in your arm bar and feel fine, is to insinuate I'm tuffer than you and that your technique sucks.
Calling my self a steel mine would be supplementing the "I'm tuffer than you imagery", by illustrating my jiu-jitsu is hard to the core, or "Hardcore" if you will. I guess I could of said "I could sit in your arm bar and feel fine, I'm an iron mine using the alloy to make steel wich is a metephor that I'm hard down to the minerals I made with, and passing through your guard like its a yeild sign"
But that doesn't rhyme well and its...well, it's just fucking retarded. And given the fact that I only had half a second to think about it..I say I made the right choice