It is indeed different for spellcasting maneuvers, but not in the way that you're seeing it.
Spellcasting is a combination of a few things:
- First determine the number of shifts needed (YS252)
- Gather the power for that number of shifts (YS255)
- Control the power gathered (YS256)
- Target the power as normal for the effect, i.e. a weapon or a maneuver (YS201/YS207)
In the quote that you reference from YS252, they're talking about determining the shifts of power required. So, if you're targeting the nameless hapless goon of the night master with Athletics +1 with your oil slick spell, your number of shifts required is three. If you're targeting the summoned skillful ninja of the night master with Athletics of Superb (+5), you need five shifts of power.
Then you compare your conviction to the number of shifts to see if you can summon that amount of power without exhausting yourself...
Then you roll your discipline vs the number of shifts summoned, i.e. if you're going against the goon, your target is three... if you're going against the summoned ninja, your target is 5...
Only then do you get to the opposed part. Your roll to control is also your roll to target, so if you rolled a +7 on your control roll, then the target would roll vs that to see the final effect is, per YS207.