Generally commented out lines are defaults, right?

Often yes, but not always. Here both the default is mentioned and a commented line is offered (I like commented lines as they show a proper format).

  # Set idle timeout (in minutes) before the connection will
  # be disconnect (defaults to 0 for no timeout)
  #IdleTimeout=30

Presumably so you can't turn it off simply by uncommenting the line. The default is defined in some source file.

Mine says

  # Defaults to true for security.
  #ClassicBondedOnly=true

(which is the patch indicated)

I don't follow your point.

I was answering yours (perhaps reading it not the way you intended) and the grand parent's "HOWEVER 'ClassicBondedOnly=true' is commented out". No big deal. This style of option and comment and commenting out the option (default or not) is autopilot common - no special intent here.