I didn't make any reference to spreads, but thanks for bringing that up. How you use correlations with spread trading depends on whether you are trading relative strength (trend) vs mean reversion (traditional pairs trading). Since bone and his clients trade relative strength, let's stick with that.Not exactly true. If there were such strict 1:1 correlation even at shorter time periods (vs longer ones) then there'd probably be less opportunity to make any money on movement of the spread itself. Obviously if you have something with a 90% correlation over 2 years but with wildly varying correlation at the month level, it's probably not going to be the most enjoyable position to hold...
Put in the simplest terms, correlation tells how often both price series move in the same direction. It doesn't tell you anything about the magnitude of the moves. So you could have a perfectly correlated pair (always moves in the same direction) and make a profitable spread trade if you correctly predict the relative strength.