Don't remember זו being used for "that" instead of זה
The words זאת and זו are feminine and זה is masculine
Why זו instead of זה?
זו/זאת = feminine
זה = masculine
why can't it be zot eino chagura shelahem?
It can be zot eina hachagora shelahem, זאת אינה החגורה שלהם. The word אינה agrees with חגורה.
Eino is masculine, Hagora feminine :)
also, because the belt is a known object (with 'the' before it) you have to add ה.
Why does שלך not work?
Because שלך is "your".