I don't know about teguixin, but T. merianae will "attack" sometimes when they're fat, slow and self conscious about it. A big fat adult who doesn't think it can run for cover fast enough will try and hold its ground and charge at who or what ever is bothering it.
From my limited experience adult Tegu's rarely puff up and posture [like Monitor lizards]. Young ones do, but not the adults. The adults will gape their jaws and/or charge.
Again, this is for merianae, I don't know if this really applies to teguixin.
What might have happened in the story provided is that the tegu, in the open, didn't think he or she could break it for cover fast enough before the guy might catch him, so he decided that fighting was his best bet for survival.