It turned out it was working, just not the way I was expecting.
Turns out you can't unindent just one line, due to the tag, you have to select multiple lines.
I simply changed my bindings to:
<binding key="shift+tab" command="unindent" />
<binding key="shift+tab" command="prevField">
<context name="hasPrevField" value="true"/>
to get the behavior I wanted.
I love the customizability (is that a word?) of this program