To cry uncle means to admit defeat or plead for mercy, but in this sentence they are acknowledging they are winning.
Context: "Neither of them would acknowledge they are winning, so it was a macho-man contest now to see who could ride the biggest wave for the longest amount of time."
Hypothesis: Neither of them would cry uncle, so it was a macho-man contest now to see who could ride the biggest wave for the longest amount of time.
Based on the context, is the hypothesis true, false, or unknown?