Coinbase is a centralized exchange (Cex, not a Dex) so it has little to do with DeFi in general.
b) Coinbase is many products. Coinbase Staking is the one that matches what was described above. Don't conflate the front facing CEX for everything they offer. No different than Amazon not being a bookstore, nor just an ecommerce platform. It is many products. The context was solidified amongst several other products with similar offerings. Coinbase's various CEX products have nothing to do with their Staking product (or the Lending one they were going to try, of Vault or several others)
b) staking has nothing to do with DeFi
Banks and brokers that offer exposure to the defi market are called CeDeFi because you could interact with DeFi yourself with no restrictions but the organizations that help you are just banks and brokers so it is a centralized company layer on top of the defi market. Those come with restrictions.
Their staking products allow you to provide them with liquidity, similar to a deposit at a bank or certificate of deposit (CD) at a bank, where they take your balance and put it in DeFi products to make a better yield for themselves.