I mean I really don't know what library they could've tried other than Skia that's suitable to build a GPU renderer for Canvas's API, and they explain why they were hesitant to try Skia. Do you have any suggestions?
Oh I read the article with the assumption that this was 2D canvas rendering, but upon re-reading it I see that it's probably about normal web content. WebRender would've been a terrible fit for Canvas, but it would certainly make sense if this is about rendering normal web content.