I mean, sure, clearly they want the giant eagle (for example) to be something more than just a beast - that's the entire point behind making this change.
What I'm getting at is, putting the giant eagle into the Giant creature category has lore implications that they may not have wanted to make. As a particularly relevant example, the giant goose in Bigby's Glory of the Giants, which has an INT of 6 (compared to the giant eagle's 8), is not itself a Giant, but is instead a type of Fey.
Just because it's big enough to be described as "giant" doesn't mean they want it to count as a "Giant".