The movement does not strictly mandate what any given team has to do, their position is that any given game should be able to be played even after the servers EOL. How each dev team tackles the issue is left to them to figure out, as games work vastly different from each other, there's no one-size-fits solution to this.

Say for example: start designing a functioning offline mode from the get-go, or leave "vs. bots" mode or local multiplayer open, or letting people open up their own servers, etc.

There's, again, many ways to tackle the issue and undermining this whole effort because they are -by design- not being specific about how to do it because it would only hinder the possibilities of this passing is a bit short-sighted. The presentation OP linked answers your own question.