The old feller was wrong. The bull's genetics don't change. If he's a calving-ease bull at 18 months, he's still a calving-ease bull at 10 years.
Late winter-born calves have the potential to be heavier... not because the cows are trying to make sure their calves have a better shot at survival(they don't have a clue, but Ma Nature does)... but because cold temperatures cause peripheral vasoconstriction(blood flow to extremities is limited) in order to conserve body heat, and in doing so, more blood is shunted to internal core organs, including the uterus... thus, enhanced fetal nourishment and growth.