Not sure I believe that. I have sailed in winds well over 30 knots and not noticed significant stretch on my halyard. Based on empirical evidence, I do not think strong winds put a large load on the halyard on my boat.
This is too easy...
Wind Load Calculator
Just b/c you don't notice it, doesn't mean it isn't happening. W/ a mainsail area of 257 sf (not that large for a 34' boat) you're seeing a load of 443 lbs on your sail in 20 knots. And you believe that the halyard doesn't see a great deal of that? Halyards have some of the highest loads of all running rigging on a boat. And they're probably the one line you don't want to stretch.