A super literal reading of some bad wording in 3.0 created an effect the authors say they did not intend and fixed in 4.0. Given the authors did not intend this interpritation a judge is likly to assume people using the licence before it came to light also did not, hence switching to 4.0 is fine. Conversly now this is widiy known continuing to use 3.0 could be seen as explicitly choosing the novel interpritation (arguably this would be a substantive change).

> a judge is likly to assume people using the licence before it came to light also did not

Why would the judge have to assume anything? The person suing could simply tell the judge they did mean to use the older interpretation, and that they disagree with the "fix". They're the ones that get to decide, since they agreed to post content using that specific license, not the "fixed" one.

A license is between two parties neither gets to choose exactly how it is interprited.

But the people suing aren't trying to choose how the license is interpreted, they're trying to prevent the other party from changing the text. If the change is meant to "fix" how the text should be interpreted (which is what you said), then they're the ones trying to choose the exact interpretation.