Whatever Meraki is doing with this, it really should be documented.
(Configuring an IP from "TEST-NET-2" was ok for the MX)
Well you aren't supposed to use test net addressing in your customer networks, so perhaps they saw it fit to use those in their internals so they would never conflict with RFC1918 addresses.
Well, it is not a customer network. It is 100% its intended use: preparing a new chapter in a training course.
Strictly speaking, it is reserved for documentation (see the RFC).
One could argue whether Meraki is simply following the standards/RFCs or whether you should be able to use it (and violate the RFC). To be honest,I don't think the forums will help you with this issue.
Oh, I think the "Documentation Feedback" forum is the right place to ask that something gets documented when a device does something specific.
I wonder if it actually is as it says - and is used internally.
The most likely usage is some kind of internal traffic redirection. I really would like to know.