Captain America takes it after a so so fight. Shang chi is the best martial artist in that he has the most perfect form. That doesn't nearly qualify him to beat captain america who is a perfect physical specimen with the best practical fightling skills, bar none.
It's like the opera chick that was on American idol (idle). Yeah she was technically the best singer, but not nearly the best practical performer.
Ever watch ultimate fightings early days? The guys with the best kata were routinely creamed by the best fighters trained or not.