You are right about them dumping bitcoin at any time but in the long run, I think this will do more good than harm because without the involvement of large institutions, it will take longer for bitcoin to become globally popular.
Actually no, he's not right about that. Blackrock cannot simply dump Bitcoin. Their ETF mandate requires them to hold BTC 1:1 to match investor demand. The only "dump" that could occur, is if their ETF investors collectively decide to sell (and such selling is not matched by ETF buyers).