There's no "exploit". The emphasized section just says the programmer was incompetent, but by chance his rootkit still (mostly) works.
Again, this is a rootkit. You need root access through some other means to install it. Still a nuisance, though.
Actually, I'm relatively sure that in practice that's precisely what it is.
Although more familiarity with similar systems/interfaces (in the real world too) than with the actual OS/program in question.
Maybe this is regional variation, but in NZ it seems the kind of person who wants to be called a Software Engineer is exactly the kind of person who thinks everything fits into their pet methodology. Which is often some inflexible misunderstanding of OO.