This is actually caused by a minor bug in the Radio module, in that the module makes no effort to make a $foo.label$ key available for downstream modules. A fix is already in place and it will ship in the next release (3.3.1) .
Sorry I was initially confused because I misread your question reading Pulldown for Radio, and this bug doesn't exist in the Pulldown module.
For completeness, there are 5 modules that share extremely similar conventions and params. Of these the most commonly used and the most extensively documented is "Pulldown". In practice if you get familiar with the Pulldown and the various pages of Pulldown docs and examples, the same conventions and params exist over in the other 4 modules, although these other modules have less verbose docs and fewer explicit examples.
These 5 cousins are Pulldown, CheckboxPulldown, Tabs, Radio and the Checkboxes module. Of these 5, Radio and Checkboxes were both by mistake were given no $foo.label$ feature, and both will be fixed in 3.3.1.
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