The suggestion here is that YC's view is due to them not being impartial. I think a more likely explanation is that they are indifferent - their investment system has continued to allow them to 'buy low', even through-out this so called bubble, and AFAIK they only sell at maturity (aquisition / IPO), so the timing of the sales are not bubble timing driven.
If you see YC attempt to liquidate lots of their positions at once, then maybe your suggestion might ring true...
Buy low is contingent on sell high. Reaching sell high requires funding to live long enough for an exit. Constantly saying "there is no bubble" helps investors feel better about a three month old company claiming a $10MM+ valuation. "No bubble" also helps acquirers justify paying > $20MM for three guys sitting in their underwear typing on a computer for a few years.
(disclaimer tags: conspiracy, not aligned with reality, condescending, wrong numbers.)
In general, good investments should remain good investments even if there are no buyers. I'd bet this is true for startups as well. Overvaluations help, but aren't necessary.
If you see YC attempt to liquidate lots of their positions at once, then maybe your suggestion might ring true...