Smith recently spoke out about the incident ... saying the idea the fight was about $600 he owed IK after flaking on a football camp appearance is "the biggest misconception in America."
TMZ Sports talked with a respected Jets veteran on the condition of anonymity ... and he says Geno's statement simply doesn't jive with what he saw and heard that day.
"Geno just saying that sh*t 'cause he don't wanna look bad."
Our source says IK was fuming about Geno's no-show even before the fight ... and personally heard him mention the money as the two men were being separated.
"IK was clearly upset about him not showing up to the camp. IK was talking it up to everyone at the camp that Geno was gonna be there."
Here's the blow-by-blow:
"So IK approached Geno about it in the locker room. he was clearly frustrated, and they were yelling at each other. And Geno said he didn't wanna talk about it, and kinda turned around, and that's when IK sucker punched him. Then we were pulling them apart, and then IK was like, 'You owe me money!'"
By the way, if Geno is lying ... it wouldn't be the first time -- remember the plane incident of 2014?