Joshua Gay's edit looks sloppy at best: I found at least 2 clauses and a section in http://creativecommons.org/licenses/by/4.0/legalcode that look incompatible with any GPL. I've written a letter to the FSF asking them to clarify this matter. As of now, I would not consider that edit a reliable source of information.
Why he made the edit when he did? Perhaps, because that's when he checked the compatibility. He actively participates in the CC-BY-SA - GPLv3 compatibility effort - that could very well be the reason that drew his attention to that matter.
The "transitive" compatibility of the earlier versions of the CC-BY has already been well explained by apsillers in c556709:
Earlier licenses are incompatible on their own but are compatible if the work allows relicensing to a compatible version. As you can see, this is a general principle not specific to a license.