"That applies to issues of fact, but the meaning of that clause of the GPLv2 is an issue of law."
I don't know if US law has the same complication but I can remember issues of mixed fact and law turning up in an IR35 case long ago. As far as I can remember, the line ran like this:
- The judge* decided on the law in interpreting a contract
- The interpretation became a fact in the case
- It then became impossible to challenge the judge's interpretation on appeal because it was now a fact and the appeal couldn't redetermine facts.
* At that sort of tribunal the actual title might not have been "Judge" but the function was essentially that.