ESTps may initially come off as introverts because they tend to be very laid-back and low-key, as Liddell is (I believe this oft-misunderstood ESTp trait was discussed in another thread). ISTjs are much more serious, reserved, and strait-laced; they tend to reveal less of themselves and are more oriented toward work and responsibility than sensation-seeking and free-spiritedness. This is not to say that ESTps don't work hard - they do - but unlike many ISTjs, they don't live to work - they work to live.
ISTjs are also very stubborn and often prefer an argument over a leisurely conversation. Liddell is very jovial and sociable (you should see him with women) - he's laughing and smiling in most of his interviews and is incredibly difficult to intimidate/perturb because of his even-tempered, easy-going nature. Also, I sense that he has a disdain for many J-type activities (he has a degree in accounting - a profession suited to ISTjs - but says that he would never want to use it) because of his uninhibited ways both inside and outside the ring. Lastly, his demeanor on Conan epitomizes the Socionics ESTp description:

"When in conversation, ESTps always show self-restraint and tact. They generally interact in an open and friendly manner, always showing positive emotions."

He's also slightly cocky (but then again, he has a right to be), hedonistic (after winning his title, he apparently partied in Las Vegas for three days non-stop), and desirous of attention (look at the mohawk, people), so in essence I think he's very much an Se-dominant extrovert.
But I could certainly be wrong.

FDG, what do you think? I'm in need of your ESTp-ness on this issue . I'd also love to hear from any ISTjs who might be interested.