I wonder if they looked at Cue [1], a data language that I think hits the sweet spot between readability and power.
Cue is a spiritual successor to Google's Jsonnet, but corrects the latter's design mistakes. Cue is quite simple, but also extremely readable. It looks ideal for the sort of data-level validations that Polaris wants to do, without having to express a DSL in YAML (which should be an antipattern by now) to do it.