Numerically that hasn't really held true. I've looked the data and both XMR and most other alts are highly correlated with BTC (and each other). XMR is actually more toward the "more correlated" end of the spectrum.

There are maybe some exceptions but for the most part if you are buying or holding any in particular, you are either betting on a decoupling in your favor (over some time horizon might be short or long), or hoping that the relatively small residual is in your favor (of course both can be the case).