This is not 100% true because Cex are created for this purpose where you can trade and make deposit and withdraw more quick than dex. When you deposit your fiat and buy btc in exchange, you have invested in Bitcoin. The only problem with Cex is security because you have no private key of your wallet. If any problem happens with exchange and they stop to withdraw funds then you cannot withdraw it. buyer of BTC in exchange will also be called investors but we will say that he is not true holder until withdraw it to own wallet or sell btc ti convert into fiat
Agree, even if our bitcoins are on centralized exchanges, we already own bitcoins and can be called investors. Although the concept of bitcoin is decentralized, it does not mean that if we store it on centralized platforms, we do not own it, as we can withdraw it to our wallets whenever we want. But storing bitcoins on CEX is really something to stay away from, as we never know when they will crash. CEX means trading and exchange and should only be used for buying and selling. Not a safe place to store bitcoins.