I think this is especially confusing since the comment says that the primitive always fails, and then the expectation is that the Smalltalk code that follows is executed instead. But that code does not do what the method actually does
I disagree. It does exactly what the method does (it *is* the implementation of the method) unless the stack is unwound. Yes, the comment could point the reader to Context>>#resume:through: which runs the ensure: & ifCurtailed: blocks on unwind. Bit otherwise ifCurtailed: is not somehow magically not executed. It is what it is ;-)
As I said earlier, ifCurtailed: only evaluates its argument if a non-local return or exception return is taken and the normal return path is not taken. See Context>>#resume:through: which runs the ensure: & ifCurtailed: blocks.
Can I confirm that your dissatisfaction is with the comment? Or do you really think the ifCurtailed: method does not execute verbatim in the absence of unwinds? If the former, you're welcome to submit an improved comment. If the latter, you're mistaken.