Of course that is the way it works.
No way even a Democrat polling organization is going to release or agree to have released a poll where people know they have sampled 90% D if the poll is to become public. If they look like absolute idiots with predictions wildly out of touch with results NO one will hire them, Dims or Republicans.
But, obviously, they will agree to generate and publish samples that are +10 to +13 Democrat -- because that could happen...even if it is improbable.