Playtest responders are definitely a tiny sliver of a minority of DnD players. In my experience, the majority of DnD players don't even know what Wizards of the Coast is, much less keep up with daily news about the game.
The purpose of the surveys wasn't to make game design decisions based on the opinions of a representative sample of the player base, it was to gauge whether the ideas they were planning to implement would cause massive riots or whether they would go over relatively well. As far as taking a temperature check on whether their ideas would go over well, they surveyed properly by asking the section of the player base most likely to have strong opinions (and most likely to make those opinions known)