It doesn't help that the word "Wordpress" is used for all sorts of different, but overlapping things: wordpress.com for his private WordPress commercial host competing with WPEngine, wordpress.org for the WordPress Foundation he also controls, WordPress describing the open source GPL licensed project.
Google makes things much simpler to avoid misusing their trademarks by splitting Chrome for their commercial product and Chromium for the open source project.
I like the comparison to Chrome and Chromium. I don't know the timeline of the whole wordpress IP. At this most it has arguably undergone genericization. That's why I think it's complicated enough that there is merit to arguments on either side.
No, WP Engine has been highlighting WordPress and using the word in a way that it did make a lot of non-technical people believe that WP Engine was 'WordPress'. In that, Mullenweg has a point.