The only time there can possibly be an issue is if the rules introduced by the author in that particular story are contradicted. And even then, there's a ton of wiggle room if the circumstances are even a bit unusual.
Like, if you don't think it's interesting for the protagonist to be hyper competent, that's fair, that's a reasonable opinion, so just say that instead of trying to imply that they've violated some inviolable logical principle with their imaginary story about people who can cast ice spells.
I think the critique is valid.
The author literally did just establish that you aren't supposed to be able to copy magic by looking at it. She went out of her way to establish that it does in fact contradict this universe's rules, because contradicting universal rules is what makes the protagonist so Special and Cool and Unique.
Beyond that, even if rules tend to vary between fantasy settings, there's still a general central concept. You would feel deceived if The Hobbit spent the entire book talking up Smaug, a legendary dragon, and when they finally visited his lair, they discovered a rabid Chihauhau. You generally expect dragons to be large, powerful, reptilian, breathe fire, etcetera, etcetera. Individual traits can be broken and still remain recognisable as a dragon as long as the rest remain faithful, but if you forego too many iconic traits then your readers will feel like the thing you're telling them is a dragon is not a dragon, even though you say it is a dragon and it's your universe so Chihauhaus can be called legendary dragons if you want them to.
...that was really long-winded and kind of tangential to my point, but anyways, one of the things I'm getting at here is that the central concept of magic is that it's not physical. Regardless of story, it's the opposite of physical. You cannot see the process behind it, that is quite literally what makes it magic. Magic is virtually always associated with intelligence or spirit, things that are invisible to the naked eye. So no wonder it tickles somebody's suspension of disbelief wrong... how are you supposed to copy something by observation when you are incapable of observing somebody's mind or soul? If the setting's rule of magic was that ice magic depended on how well you yelled out the word "Freeze!", that would be one thing, very silly in its own way but at least observable. As it is this just looks like a nonsensical cheat skill to my eye.
(I want to go on record and state that the length of this reply is not proportional to how much I care to argue about this particular thing, but rather, suspension of disbelief and worldbuilding are very dear subjects to me and I simply enjoy thinking about them.)
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