It is not. Think about it. Diversification ensures that if one of your assets degrades in value, you have an unrelated asset that can still do well. Back to Alphabet, if ads revenues disappears overnight, Youtube becomes a dead project. Simples
At least they have alternate ways of selling ads, though. For example there has been a lot of talk about how their search business ads are threatened by LLMs that answer questions directly instead of giving search results that include paid placements, etc. But even if that happened, it likely wouldn’t affect YouTube ad revenues much.
Is it? If some new thing came along tomorrow that made Google's ad exchange obsolete, they could still sell ads on YouTube using whatever the new thing is. Or if YouTube became untenable, they'd have the ad exchange.
You two are nitpicking over "ads" vs "ads exchange" without saying it or talking about it meaningfully