I dont think thats a good enough reason to call it a bear market, there might be other reasons but that supply coming onto an exchange alone is not enough.
Mt gox was a known negative over 4 years ago. I never heard of people believing these coins had been written off, no longer available. That supply was there, not apparent on an exchange perhaps but its part of the price previously if not the liquidity.

How I would read this news and price move as, mt gox regulators cashing out its assets and would be a temporary change to supply and demand.     It might not be perceivable as a trading signal but its fairly straight forward to call it a buying opportunity.  The weakness cause is known and accounted for, its also temporary and minor to the overall market movement.

Were the silk road coins a far larger amount at the time.   It wasnt really a good reason for the market to go down long term.  An actual shift to confidence has to occur for it to matter in market making terms I think.