I have no idea who the “authority” is who you received that response to, but it’s the same as buying lettuce, looking for bugs and then eating it when you don’t immediately find any. This is something some modern Orthodox people do, and it’s potentially worse than eating chazir, as every bug carries 5 laavin with it.
As for how common it is, it’s happened to me personally a few times. And i only buy cheap stuff. You never know when they’ll decide to use linen in the collar, or in the lining if it’s scrap material.
Ask the Mikdash melech shaatnez lab in z. Bermans on coney island avenue for dozens of examples of shatnez found in everything from sweaters to scarves; anything that has wool. They have examples on their walls. And if you think they’re nogaya bedavar, take a look at their prices and get to know the people involved; they’re some of the most selfless people I know. They’re genuinely determined to save the tzibur from shaatnez.
Chazal say that one who wears shatnez does not have his tefilos answered. This applies even to unintentional transgressions. Inagine davening your heart out on yom kippur only for it not to matter because you couldn’t be bothered to spend 7 dollars on checking your suit for shaatnez…