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?
he just used a bad example (litecoin) to explain what has been also explained before. it is not about the most popular and the oldest altcoin which happens to be listed on major exchanges that trade it with fiat. it is about other 2500 altcoins, specially the big altcoins that are being traded with large volumes (get dumped on large scale too) but don't have fiat market or if they do, they don't have it on major exchanges. for example someone who uses Coinbase as his bitcoin trading platform and Binance for his altcoins, has to sell for bitcoin, then transfer that bitcoin to coinbase and sell for fiat. and that is why bitcoin drops when there is a gigantic altcoin dump going on.
the reason why it is not dropping as much is be cause bitcoin market is huge and it can absorb that kind of crap and also because not everyone exits to fiat, they dump their shitcoins to get their valuable bitcoin back not their fiat back.