A "Reader" could be a peace of software, that is completely free to use, but obviously would have very limited functionality in editing. A free light Version of Affinity Publisher would make it easier to exchange work in progress with a customer for example (that doesn't want to buy their own copy). Even thou Affinity Publisher is not that expensive, a "Reader" could have a really easy to understand and simple UI and just enough editing features to work with the customer on the same projekt together (for example could the customer edit text within the .afpub).
There would be absolutly no reason to have an extra xml-format that is not compatible with other software, when a light version of Affinity Publisher or "Reader" could just open the .afpub. And if it is really just for "reading", there is no reason to not just send a PDF of the work in progress to your customer. So a Reader with limited editing funcionality is the only thing that makes some sence...
I personally woul not see that as a Feature on my wanted-List, but I also wouldn't see it as completely useless... anyways - it would not be a real replacement for idml-export, since the reason why people would want that is compatibility with inDesign. And I dont think Adobe would implement an import option for an Affinity xml output.