This is something commonly quoted, but it’s not entirely true. In minuscule amounts (ie. what is when wine is “normally” mousy), this is very true: the wine smells fine, tastes fine and only in the aftertaste you get that horrible smack of mousiness.
However, I don’t know whether it’s when the levels of THP are higher than that, or if it’s some other compound (mousiness is actually not caused just by THP (which itself can be three different compunds: 2-acetyl-3,4,5,6-tetrahydropyridine, 2-acetyl-1,4,5,6-tetrahydropyridine (both abbreviated either ATHP or ACTPY) or 2-ethyltetrahydropyridine (abbreviated ETHP or ETPY)), but also by 2-acetylpyrroline (APY)), but I’ve tasted enough natural wines and sour ales where you can actually smell and taste mousiness on the midpalate, not just retronasally in the aftertaste. Those wines and beers are few and far between - I’d say 1 in every 25 to 50 mousy beverages - but they still are there. There are wines that smell like rancid nuts and sesame seeds already in the glass and there are wines that might smell just fine, yet having that unmistakable Cheerios taste appear on your palate already before you’ve swallowed even one little drop. I can actually go and dig multiple tasting notes on wines and beers like these, if you want to experience them yourself!
And FWIW, THP is also a molecule that can appear during Maillard reactions - and I’ve understood it is quite typical for an unfinished beer to show a tiny bit of mousiness early on. This character is believed to stem from Maillard reactions that can happen during mashing, but these qualities are also believed to disappear during bottle refermentation ie. carbonation. I don’t know how the chemistry is supposed to work here, but I’ve heard from several of my friends who brew their own beers that their beers can be subtly mousy for a short while, then it disappears.
Finally, I remember reading how brett can actually metabolize THP (and/or other molecules responsible of THP). This could explain how some wines (that don’t have meaningful amounts of brett) never see their mousiness go away, while others actually seem to resolve their mousiness. While I myself haven’t seen mousiness go anywhere (because I try to steer clear of mousy wines), I’ve heard from my natural wino geek friends that some wines that have been mousy upon release turn ok after some aging. However, I’ve been told this has taken anywhere from 5 to 10 years, not just a few months.