ETF Model, momentum and correlations rules?

I understand how this works but it seems to be missing one essential component: the expected return of each ETF. As I see it, the three essential variables in portfolio weighting are expected return, correlation, and volatility. If the expected returns are all equal, then this formula, which takes into account the other two variables, should work well. But if they’re even slightly unequal, does correlation make a lot of difference? I’m still trying to figure out the answer to that. If you have any insights along those lines or recommended reading, I’d love to hear.