My understanding about accessibility of a web page is, that all its contents must be accessible without a mouse.
So was the Map Kitchen examples written. Take categories example and you can open any info window by [Tab] key, check the checkboxes by [space]. ([Shift[Tab]] to go back).
Accessibility is in the hands of the developer who writes the code for a map page. It is not built in Google Maps.
Patrick H. Lauke from Opera does not accept my opinion. We had a short public discussion on the forum and our opinions are:
Google Maps control elements (zoom, nav-buttons...) should be keyboard accessible
Those functions are easily made keyboard accessible by the page developer but they are not important. The contents must be accessible. There should be a method to open any info window without mouse.
Patric has made great work with a script that converts Google Maps control buttons keyboard accessible