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?
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
Shitcoins, despite their multitude, are inconsequential
Even if their numbers are in the thousands, their effect on Bitcoin's price is likely in the thousandth of a percent. In any case, this effect wouldn't be greater than the margin of error provided Bitcoin is actually used as a conversion device (which itself adds another level of doubt). People with shitcoins use exchanges where you can exchange them directly for fiat without the hassle of multiple conversions (for example, Cryptopia)
If anything, it is Bitcoin's price moving a couple of percentage points with shitcoins crashing like there's no tomorrow