That's always been a rule of thumb rather than a hard rule. In fact, if you read the FSN line that mentions this in JP it's not even close to as "absolute" as the English line makes it seem.
The idea is that a greater mystery will have a generally higher chance of negating/winning out over a lesser one, like how if you try to shoot a phantasmal beast with your shitty modern magic then it'll just dissipate and not do anything. But it's not absolute in the sense of your spell is 100 years old and mine is 101 so mine always wins. Perhaps if it was the exact same spell and the only thing that differed was the "accumulated mystery", but otherwise you still have to take all those other factors into account as always. And Fate has practically always been about advantages winning out rather than just raw power, and it's the same thing here. Hell, even the actual scene in FSN this is from doesn't even paint the scenario as "pegasus is older so it's gonna win", it's just describing why this magical horse is strong.Originally Posted by Fate, Day 10: Sword of Promised Victory
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When you think about it, most of the stuff from FSN that people think are "hard rules" are just these kinds of rules of thumb, even ignoring "nasu loves to break his own rule". The english lines just often make things sound so factual and black and white.