And so we must rely on v4 addresses to cater for v4 only connected hosts. This is the point the article is trying to make - for a transition to actually happen there needs to be interoperability between v4 and v6, that is v4-only clients need to talk to v6-only servers, and v6-only servers need to talk to v4-only clients. That is a fundamental requirement for a transition plan which provides a straight-forward path from v4 to v6.