So, it's like having bitcoin but not having bitcoin. In my opinion, anyone who doesn't do self-custody only tries to benefit from price increases and not from actually holding the coins.
You own bitcoin only if you own its private key or mnemonic seed. Without private key or mnemonic seed, what you have only a visually number on the screen that you have risk of never be able to access it in future.
Accesing it or not, it depends on the custodians that own private key, and whether they approve your withdrawal request or not. To be more exact, you still don't have acces, but they will access those bitcoins, and proceed it on behalf of you, and you will ONLY have access to those coins after receiving it from the exchange to your non-custodial wallet.
Again, the big condition at the end, is you must store your withdrawn bitcoin from that exchange to you non-custodial wallet, but if you store it in another centralized exchange, you again don't own those bitcoins.
Bitcoin ETFs are like this and many Bitcoin ETFs are even not self-custodians but they hire Coinbase doing this as a third party custodians.