Android P is reportedly already in testing for Chrome OS according to some recently discovered wording from a commit over at the Chromium Review. The text in that commit reads Hook up _P tests to android-container-pi builds. There is little here to go off of in regards to what the testing includes, but it does highlight that Android P testing for Chrome OS machines like Chromebooks and Chrome Boxes is already underway.
Worth noting is that Android P is still in testing with Android devices and has so far, only released as the first developer preview build, with a few more builds to go, so this showcases that Google is testing P with Chromebooks quite early. Also worth noting is that Oreo-based software for Chrome OS has yet to release and Google is already making headway in testing with the software versions that comes after Oreo, which may suggest that either Oreo-based software for the platform is close at hand and could reach public machines in the near future, or that Google plans to simply skip it and continue with testing Android P software on the platform until it's ready instead, and it would appear that it may be the latter.
Why Google would choose to skip an entire version is unclear, but perhaps it feels it would make little sense to release an older version of software for Chromebooks when there is already another one on the way and due for a release on smartphones by the end of the Summer or beginning of the Fall. Another interesting thing to consider is that if Google decides to push Chrome OS software right past Oreo and into Android P territory, that doesn't mean that Chromebooks would miss out on some features that may have been planned for an Oreo-based build of Chrome OS as Google would likely just issue them with Android P instead.