I didn't see anything about the pea not freezing up in freezing environments. I've never seen any manufacturer make that claim.

Here's how it happens: You blow the whistle in freezing temperature; the condensation builds around the pea and eventually causes the pea to become too large or to stick to the whistle. Whether or not it's a real danger, a pea-less whistle does not have this problem. So, there are other options if you want peace of mind. I have pea whistles, but I don't carry them in the snow.