You're right, he actually generates analysis on public polls rather than any polls himself. He would be out of a job if no one thought they were valid, and further, he actually did do some polling of his own (via FiveThirtyEight) in the now distant past. It's perfectly reasonable to think and believe that campaigns will publicly release "internal polling" results that are fabrications to develop a public narrative, but this doesn't actually address the question of whether real internal polls are more or less valid than public ones.

Here's the thing -- we ALL know that public polls are almost entirely Left slanted and intentionally erroneous to some degree, not even speaking to their other inherent flaws. Which is to say, they're unreliable just on the face of them. While internal polls could suffer the same problem, hitting on Silver's claim of "principal-agent" and "true believer" bias, there is a much, much stronger incentive for candidates to operate on precise internal polls. After all, there is a tremendous amount of money and power involved, not merely ego stroking, and the people backing these candidates apply even more pressure for them to use good internal data to get the desired outcome -- won elections that solidify regulatory capture, corruption and all the other things being paid for by these external parties.