He (Mark) wasn't a "top flight tester" at Boeing.Anyone joining an airline has to join at the FO level due to seniority rules and work up.
According to the article (I know...) this person was responsible for the major redesign of the MCAS system based upon his input. He was also (again according to the article) the point man for FAA documentation on the issue, and if I read it correctly he is the one who convinced the FAA to allow Boeing to remove the mention of MCAS from the flight documents. You cannot convince me that an organization such as Boeing would make such dramatic changes based upon anybody who was not at the top of the flight test food chain.
To be honest I do find the sequence described in the article a little hard to believe; they imply that it was test that came up with the engineering solution which is pretty rare in my experience, but I don't work with aircraft. I feel that there is a goat named "Scape" somewhere in this story.