Hey, turns out I was right in my comment, more or less!
>This guide has been created as a resource to help battlers understand the many magic numbers that define the game ~ Smogon
A magic speed number (or magic stat) is a number that is an important benchmark for a Pokémon to hit. For example, a Pokémon with a Speed stat of 198 will outspeed positive-natured base 130s after 2 boosts (e.g. Agility). This is important if you want to outspend something like Gengar.
TL;DR – A "magic stat" is a stat number that allows a Pokémon to reach a specific benchmark.