To let bygones be bygones is to stop focusing on something that happened in the past, but in this context the speaker is willing to hold a grudge and not forgive
Context: See, I was willing to hold a grudge and not forgive till they took things too far.
Hypothesis: See, I was willing to let bygones be bygones till they took things too far.

Given this context, determine if the hypothesis is true (Yes, no, or unknown)?