Even if she's been asked about it numerous times, context is key here - how was the production of the music video shoot handled? How much input into that scene or clip did she actually have, or provide, when it was shot? How long did she hold the phone for when shooting? If the answer to many of those is little to none, then I don't expect her to remember shooting that scene. To then be asked about it numerous times years later would be surprising to her, especially if you don't necessarily know that it's been going around as a meme (not every celebrity enjoys paying attention to what's said about themselves online - we don't know whether or not she does, but I won't assume she's up to speed on everything written about her).
It doesn't impact her life at all, it's just noise on the internet to her, so why should she care?
I mean, that's actually a great question for you - why do you think she has to care about what Excel is, and why do you find her not caring about it so egregious that you would call her an idiot? And further, is it not OK to be OK with being ignorant to some things you just don't give a damn about?
Edit: Can people also not be flippant when an otherwise throwaway moment in their life is suddenly seized upon by hordes of people, some who call them an idiot for it? I don't think I'd fault someone for that reaction, myself.