It's not clear to me it even was a threat; the context of that comment is entirely unclear. It's possible the couple just asked what could be done about the situation and the council responses with "well, we could put preservation order on it, but you'll be on the hook for protection then".

Given the later "it's their property and right to do with as they want" comment, I think something along those lines is more likely, but hard to be sure.

The reporting on all of this has not been of stellar quality.