I don't see why this is plus que parfait. This refers to an action completed in the past. Seems to me "je te l'ai dit" would serve just as well or better.
Freeform Writing Exercise B2
I think the point here is likely that “I told you so!” implies that what you said was prior to another ‘event’ - in this case that could be “I told you so, before you checked the weather”. Hence plus que parfait.
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