SQL Server - Is there a limit to the number of Distributed Availability Groups that a primary replica can participate in?
You are misreading that document. In the document and image you are referring to - there are actually two distributed Availability Groups in the image you refer to. each one containing two Availability Groups.

A distributed Availability Group is distributed between two (and as far as I now - only two) Availability Groups.

You can have multiple secondaries in an Availability Group and they can span data centers, so depending on what you are trying to do, you may not need a Distributed Availability Group to do it.