I would concur with your thoughts, Ron. We had a wall in our bathroom where it was leaking behind and mold showed up through the wall. I believe if the wall is subject to extremely wet conditions, it can mold like that on the front. When we had a flood in the basement, the unfinished gyproc began to show black mold before we extracted all of it. It's a very unhealthy situation to be exposed to.
From the flooding we were exposed to in 2014, the government people being sent out to give information did explain that bleach is not effective on mold. There are only specific cleaners that can effectively clean mold.
However, from the severity of the specific case you posted about, it sounds like it may need to be dealt with at a construction/restoration level. I believe I'd let experts deal with that.