requires no extensions in the spec
That proposal doesnt require an extension to the spec. It requires a group to follow another group, which is definitively within the ActivityPub spec. The proposal above is written as a FEP (Fediverse Enhancement Proposal) which is the agreed upon way to propose new behavior in an interoperable way.
no changes in the server side
But it takes changes on the client side. One is not inherently better than the other. Also, doing it client side means you have to duplicate the work for every client. Doing it server side means it works for everyone.
easily prototyped/tested
Every fediverse platform already supports following Actors. That's part of the spec. Handling follows for groups is just as easy as for users.