You are not missing anything, changing this option is not implemented for this tool at the moment.
That being said this tool is entirely defined in an XML file where this aspect is just an attribute so you could change it from there in the meantime.
In the program directory locate the DrawingTools > Custom > "8 - Angle to horizontal.xml" file and open it in a text editor.
In the Data section locate this:
<Angle origin="1" leg1="2" leg2="3" signed="true" ccw="true" supplementary="false" tenth="true"/>
You change the attribute `signed` to `false` to get the unsigned angle.