"Looks like polymer templates are not valid meteor templates."
Seems, it's doesn't matter at all, because Polymer components are very isolated, defined statically (for Meteor it's jsut new HTML tags) and are used only on the client. Polymer component has own interface to configure it. So, I'm not sure, but it seems Meteor templates just passed the data to Polymer component via his own parameters (attributes) and forgot about it. After that Polymer component do all their work.