Ah. Yes. It’s really badly worded (clearly!)
The API does support OAuth2 access token authorisation, which is what our website itself uses for the only filing available on the new platform, the change of registered office address. This API is not available publically, as it is an alpha spike implementation. The OAuth2 authentication is also currently alpha.
The (bad) documentation you refer to is a hang-over from when we thought OAuth2 authentication would be available in the (current) beta release: the API’s you refer to do not need OAuth2 authentication. However, if you’re not using OAuth2, then you must use an API key.
Like I said, really confusing. Sorry.
We are re-writing the developer information and will be releasing the Swagger specifications, as discussed in: