In my understanding you can sell mined Ether on Coinbase but you cannot use your Coinbase address as the recipient of mining rewards directly from a pool.
In my opinion, it should be more explicit, but the part that is important is Do not send ETH mining rewards or ETC DIRECTLY to this address. Meaning, don't point your pool to our address.
Even though Coinbase does not explicitly prohibit using their address as the recipient of mining rewards in the User Agreement or in their ToS it is better if you make a new Ethereum address and send your rewards to that address and send them later to Coinbase or any other exchange. Alternatively you can just point to an exchange that supports receiving mining rewards.
According to a thread in 2014, it appears to be related to tech. The reply from support was the following:
"Payouts from BTC mining pools are supported. However, newly mined BTC will take 100 confirmations before changing from a pending to a complete state. Mining payouts from ETH mining pools are currently not supported unfortunately.
We are working on supporting this, but significant modifications to our wallet software is needed to fully support all the ways balances can change with Ethereum (Eg. mining rewards, uncle rewards, contract invoked txs)."
Link to the relevant thread: https://community.coinbase.com/t/mining-ether-to-coinbase-wallet/12546
My suggestions would be get in touch with Coinbase support and ask them directly about this feature or any doubt you have or look for another exchange that supports receiving Ether directly from a pool if that's what you're looking for. Given that you already have Ether in your local computer you should be fine by just sending Ether to Coinbase.