In my opinion you are correct unless there was anything else in the proposal or modifications. Ask your IP for clarification in writing of where they get their reason for the extension as this may be a genuine error.
I have just checked our proposals & it says if our share of the equity is 5k or above then we will have to extend the term or try & remortgage. The 85% LTV IS also in there but nothing about the 5k being after the 85%. So i am thinking tha the 5k is at 100%?