This seems like a poor choice of technology rather than an outright nefarious decision. Polymer [0] uses the shadow dom api [1] extensively, but shadow dom v1 still isn't fully implemented across all vendors and shadow dom v0 is deprecated and slated for removal in April 2019 [2].
This feels more or less like some premature dogfooding of Polymer. It probably would've been advisable to serve the old version until modern non-Chrome browsers caught up, but Shadow DOM v1 will be implemented eventually so my guess is they made the decision to transition those users now.
[0] https://www.polymer-project.org/
[1] https://www.polymer-project.org/3.0/docs/devguide/shadow-dom