I'm from the black the base plate camp and apply blacking every two years to my boat . In this case I suspect it is relevant that the microbes haven't attacked the sides because they were blacked . I also believe that the reason most people do not black the base plate is because it is a pig of a job crawling underneath and many dry docks do not lift the boat high enough to be able to get underneath . Having said that a 10mm base plate will take a while to be 'eaten through'.
In the pictures that Cheshire Rose has added it looks as if the microbes have crawled under the rubbing strake . That again highlights best practice of having the underside of the rubbing strake continuously welded.
In the past those from the no need to black the base plate camp have claimed lack of oxygen underwater as the reason blacking is unnecessary ,I wonder what they will say knowing these microbes thrive without light and oxygen.