So who did the Native American vouch for?

He also said he doesn’t know him, even though he clearly does

For almost 30 years I assumed Shep vouched for Carl (Buscemi) simply because it was obvious they knew each other in the scene where Shep goes and pulls Carl off the hooker and beats his ass, chokes him with his own belt. Carl recognized him immediately, wasn't confused about who he was, knew him by name. No question, they know each other. I agreed with AnthonySocks.

But after re-watching the movie and paying closer attention, Ant is right. Shep says he only vouched for "Grimsrud." That's Stormare's character. Shep asks Jerry what the other guy's name is- "Carl something." "Never heard of him, don't vouch for him."

My confusion comes from A) not paying attention to the names (I never knew Stormare's character was named Grimsrud until now, even having watched the movie multiple times over the years) and B) there being no discernable reason why Shep lied about not knowing Carl. It almost seems like a mistake in the script, but with the Coen's notorious attention to detail in their writing, there's just no fucking way.