Applies to any copyleft licence

Yes, you are right, in the end it is the copyright holder of the GPLv2-only work whose intent counts the most. But do you really believe that if the FSF says "oops, a specific literal reading could cause a problem for free operation systems, and that obviously isn't the intention, so we explicitly say that and make sure that in GPLv3 such textual ambiguity doesn't exist." That there are copyright holders that will say "yes, I specifically choose GPLv2-only to cause a problem for people wanting to read that particular exception language as intending to cause trouble for free operating systems like Debian"?

IMHO, unless a copyright holder explicitly tells you to ignore the stated intent of the license drafter, you can safely ignore that possibility.