"Zero correlation to the markets" is one of the things that makes Bitcoin desirable. Diversifying your portfolio is all about having things that do not move in the same direction at the same time.
It is important to have a variety of uncorrelated assets, but in the case of Bitcoin it's still quite volatile, not always for clearly apparent reasons. It seems to be developing a place for itself, but I think it still needs a few years of less volatility for it carve out the niche many people want it to have.
A lot of things were correlated then that aren’t always / normally. Bonds, stocks, gold... basically everything was sold to move into dollars. That doesn’t mean that those assets are all correlated over the long haul.