As much as I strongly advice everyone against such practice as keeping ones coins or bitcoin rather, on a centralized exchange, I still will say that a holdler is a holdler, regardless of where they choose to hold their bitcoin, there is no much difference from the investor who choose to keep his bitcoin on a self custody wallet and the one who choose to keep his one centralized exchange, the only difference is that one person has access to his or her wallet keys and as such, have full control over the bitcoin, while the other does not have access to the keys to the wallet where his bitcoins are held and as such, have limited access to the bitcoin.
But facts remains that which ever amongst this two that want to sell can sell, the one holding in a self custody wallet will have to first move the fund to an exchange where he can sell it, while the other person who already have his funds on the exchange will simply sell.
Aside the fact that centralized exchanges can be hacked, go bankrupt, turn into a rug/scam and other risks that are completely out of the control of the user, one saves money on fees when bitcoin to be sold is already on the exchange.
There is a difference between holding your coins on a centralized exchange and a self-custody wallet. When you hold Bitcoins on your self-custody wallet, everyone can see on blockchain that you hold Bitcoins on your wallet and this has an influence over the market but when you hold Bitcoins on the exchange, you don't really hold anything because exchanges store those Bitcoins on their own addresses, they can do whatever they want with those coins and if they get hacked, you lose access on your portion of coins. Exchanges also convert assets the way they want to make money, so it's not like everyone is backed 1:1.
P.S. If you want to sell your Bitcoins from your self-custody wallet, you don't need to send them to a centralized exchange, you can do it through a decentralized exchange.