When did the definition of constellation become birth-sign. Not once did GI say the word birth-sign in ANY of it's coverage. Just because birth signs are represented by constellations doesn't automatically make all constellations birth signs.
You're right, not all constellations are birth signs. But all birth signs are constellations. However, the thief, the warrior, and the mage ARE birth signs in TES. So what's your point?
This is what the game informer says:
"Three prominent nebulae dominate the Skyrim heavens – the thief, the warrior, and the mage. Each of these represents one of the three master skill sets. Each nebula houses six constellations, each of which represents a skill."
So perhaps birthsigns are still in the same way they've always been. Just under a new system. The article wasn't specific.
I'll quote myself here and you seemed to have overlooked this in my post; "I'm not entirely sure how the birth sign system would work this time around." Everything I wrote was an assumption. I never presented anything as fact.