My expectation is that this action can never appear -- this is the pre-Drydock hacky temporary action -- so something troubling is afoot.

Oh, this is actually the Mercurial flavor of this action. We have this one in the Git flavor, which is why I was surprised:

// TODO: This temporarily disables this action, because it doesn't work
// and is confusing to users. If you want to use it, comment out this line
// for now and we'll provide real support eventually.
return;

I'm probably just going to remove these wholesale, the upstream version doesn't precisely replace them but has proven stable, at least.