Well, that means if you have bitcoin you sell bitcoin, and that's it, however if you have litecoin then you sell your litecoin and buy bitcoin with it, sell your bitcoin and get fiat. Which means litecoin is losing value by getting sold whereas bitcoin stays the same because it is both bought and sold. That is why whenever people get out of crypto in general bitcoin still drops but drops a lot less then the altcoins that are getting dumped
That's a poor man's logic
You could just as well say that if someone sells their litecoins for fiat, Litecoin loses value and that would do just fine as an explanation. In other words, you will have to leave no stone unturned if you actually wanted to find an exchange that would trade Bitcoin for fiat and would not trade Litecoin for the same fiat at the same time. Obviously, it is not the case with most shitcoins, but who cares about them and their holders anyway?