We're not considering the iMac Pro an update because it's just a couple of new options and minor price cuts on some existing high-end options as component pricing has dropped for Apple. All of the previous specs continue to be offered. Same reason we didn't consider the new Vega MacBook Pro options an update last fall.
It's always a judgment call, but typically we need to see updates to core components across stock configs or an across-the-board price cut before we start thinking about counting something as an update cycle for Buyer's Guide purposes. iMac Pro lifecycle is a little hard to gauge since it's a first-gen product, and today's tweaks do suggest a full update isn't imminent, but can't really count this as a data point.