I have reached out to the Identity Service team to clarify the documentation.
DAP alpha operated with Canvas ID and secret pairs. Canvas credentials are no longer accepted by DAP, in any form. DAP alpha used Canvas credentials because at that time, DAP only provided Canvas data.
DAP beta operated with client ID and secret issued by Instructure Identity Service, which were concatenated into a single string. This single string had been called API key, which was a misnomer.
DAP client library accepts a client ID and secret, which are concatenated internally when passing to the authentication service. Since the concatenation is an implementation detail, and client ID helps in identifying requests, we prefer specifying them separately.
I hope this helps give some context.