Not a myth. It's true.
In general, drinking things from a straw, sucking on hard candies, chewing gum, and eating too quickly cause gas. They all cause gas for the same reasons, too: you are accidentally swallowing air.
Swallowing some air is natural. But, you swallow more air when you drink from a straw. You can't really help it either, that's how straws are designed. Either you ditch the straw or burp to relieve that gas.