https://support.google.com/accounts/answer/14012355?hl=en&sjid=6320154799544542694-EU

Describe: Manage data in your Google Account: Third-party apps or services may request permission to edit, upload, create, or delete data in your Google Account.

For example:
    A film editor app may edit your video and upload it to your YouTube channel.

So, yes, is possible to give permissions to an application, to write in someone else Youtube-channel. Even if is a potencial security issue, Google warn about this permissions, giving the user the hability to manage those application.