"If non-catchability is an inheritable trait, then why did selected fish that did not prove vulnerable to angling (were not caught) have offspring that were in fact vulnerable to angling?"

The point is over time the catchability of the population of LMB became lower and lower. If the % of catchable LMB decreases from 60% to 20% you have a problem. It is insignificant that some few offspring are catchable. That is even a decreasing % over time as well.