"Scale" in this case refers to "Scaling with stats" -- or, put more simply for this case, "How the Holy Paladin's ability to heal improves for a given increase in a specific stat" (Which are acquired from stronger gear, or temporary boosts, such as are being discussed in the guide).
While I can't speak to the specifics about healing as a Holy Paladin, the implication is that the effect of this ability (the bonus Intellect) is less useful to a Paladin than another stat -- such as Critical rating or Mastery.
Unfortunately, there is no general way to determine this, as each class is different, and often that there are multiple potential interactions between spells, abilities, buffs, etc. to truly account for a "Standard" situation.
That doesn't mean people don't give it a try, and the folks over on Icy-Veins, for instance, have gone after the problem with more spreadsheets than you can swing an arcanite reaper at.