There will be some mixing of fresh air through the snorkel but that's a small opening and a long tube. It will still mostly be the air you exhaled. For a trad

There will be some mixing of fresh air through the snorkel but that's a small opening and a long tube. It will still mostly be the air you exhaled. For a traditional snorkel that's a small volume relative to a breath but not so much for those masks. Plus, if you're diving under the water a traditional snorkel will be completely purged while these masks will retain the air.