If you take a look at last changes, list children are no more converted into listitems.
Here you need to view List widget only as a component designed to managed repetitive content as described in the renderer attribute.
Because the philosophy of Kuix is be flexible while staying simple, I think it woulds be wrong to add a "type" attribute to the list.
In this case a list is not more a checkbox list than a radiobutton list. Because of the special management of radio buttons, the dedicated radioButton list is the RadioGroup which extends List class.
That's what the lastest modifications propose :
for checkbox list :
<list>
<_renderer><![CDATA[<checkbox>...</checkbox>]]></_renderer>
<_items>@{items}</_items>
</list>
for radiobutton list :
<radiogroup>
<_renderer><![CDATA[<radiobutton>...</radiobutton>]]></_renderer>
<_items>@{items}</_items>
</radiogroup>
But you're right that a all in one list would be easier to use and understand, but less flexible.