No, it's not. A steem app relies on the steem blockchain to store its data. By your incorrect definition, if coinbase were to allow you to send and receive Steem it would be a Steem app. As for logging in using steem... that is only relying on Steem credentials for verification. This verification process is merely a way for you to be identified, there is no genuine Steem interaction, just a simple hash decryption.