This loophole isn't really closed.
Second home owners can still register for business rates instead of council tax and then avoid paying anything at all if their business comes under the exemption level.
The difference is that the government have announced a review to work out how people can prove that they let their properties out for more than 140 days. The current system is that they are given the exemption if they just say they intended to!
So, nothing's changed much really.
Labour wants to charge all second homes council tax and close the business rates exemption loophole properly. We would like to charge an extra council tax levy for people who let their second homes sit empty and ringfence it to spend on council/social housing in Cornwall.