Can a wallet be completely open source and still be centralised?
Maybe a better term instead of "centralized vs. decentralized" would be "self-custodial vs. custodial".
"Self-custodial" would mean that you'll own your private keys, while "custodial" would mean that someone else except you is managing the private keys for you and you are not owning these private keys. Classical "not your keys - not your coins".
The term "centralized vs. decentralized" is better suitable for a comparison between Uniswap (Decentralized exchange) and Coinbase (Centralized exchange).
While open source (code is publicly available) isn't directly related to "self-custodial vs. custodial", there are still wallets where:
1.) you'll own your private keys but the wallet is not open source (Exodus wallet) = self-custodial + closed source
2.) you'll own your private keys and the wallet is open source (Electrum wallet) = self-custodial + open source
While I would go for 2.), point 1.) is still by far better than a centralized exchange, where you don't own your private key.
But 2.) is clearly my choice and all Blockchain experts will agree here.