EzekielRaiden
Follower of the Way
The "in theory" here includes the possibility that the PC had been built differently, e.g. choosing a different Wizard tradition, playing a different class, or even being a different race or physiology.The idea that PCs should (in theory) be able to learn the skills and abilities of NPCs doesn't make sense even if you played NPCs with full PC rules. A player character illusionist wizard can never learn the powers that other wizard subclasses grant, no matter how hard they try, barring house rules. Same thing with a battlemaster fighter observing the abilities of an NPC rune knight fighter. This mythical in-world consistency with regards to PC vs NPC abilities has never existed and will never exist even if you build every single NPC with the PC rules.
Would you allow an NPC caster to counterspell a PC druid's wildshape or a PC conjurer wizard's benign transposition or a PC cleric's various channel divinity powers? They're all magical abilities after all and very similar to spells even if they aren't spells. Or vice versa, if an NPC possessed any of these PC class abilities and a player wanted to counterspell it?
Don't get me wrong, I also think the logic is stretched thin, since this relies on the entirely un-diegetic "you choose how to build your character" stuff. But there is at least a response built in for it.