I wonder how long it will take until ZOS realizes that the majority of players does not understand that the "message rate" has nothing to do with chats and finally changes that error message.

The servers have a flood protection and will kick any client that exceeds a certain amount of data exchange with the server. This includes every kind of data, from ability activations to position changes (a.k.a. walking) to setting changes (hotbar load-out, equipment changes, game setting changes, etc.). The error message only tells you that you exceeded this limit.

Reasons for this are legion, one being packet loss which prompts the client to resent all data packets lost at once, another being addons or macros that execute many commands in quick succession. Lag is another one, as rightly said above, and probably the most common one. Imagine it as a traffic jam on the data highway, your data packets are held up somewhere on the way to the server and once the "obstruction" is removed, all of them rush in to the server at once.

TL/DR: "Messages" in this case refers to data packets transferred from client to the server, not (only) chat messages.