Not eager per se, but a) the explanation, while plausible sounding, is very hand-wavey, and b) the literature already reports them to be strongly clustered in phylogenetic groups.

E.g., from McNab and Eisenberg, "Many of the species with the largest brains relative to the mass standard are arboreal, most notably xenarthrans, carnivores, primates, and tree squirrels."

The primates and the carnivores are already known to have their own allometric series which might be confounding the arboreal effect. Tree squirrels, I don't know. But arboreal xenarthrans -- they're sloths! Like you said, it's really hard to argue that they have a larger brain than predicted because of their enhanced three-dimensional mobility.

So you have two orders where developmental allometric differences might be confounding matters, one order where the standard explanation makes little sense, and tree squirrels.

(Eisenberg did look within smaller phylogenetic groupings for evidence of the arboreal-brain size connection -- it's in his possum paper referred to in McNab and Eisenberg -- and while he found a relationship, it was hard to disentangle from other factors.)

So it's an Occam's razor sort of thing.