We need to understand the context first.

@OP said it's a Youtuber who trade Bitcoin, so we need to highlight trader, of course trader only care about the short term gain. if we talk about trading, it's correct because when altcoins rally, it's not Bitcoin price will dump, but you will miss the big gain in altcoins.

So, most likely he advise to sell Bitcoin and immediately buy altcoins because he think altcoins season will happen.