The distinction is absolutely significant. It's all about expectations. The originally promised "HL2:Episode 3" didn't need to be some huge revolutionary product, just a satisfying conclusion to HL2 and its episodes, while setting up the scene for the next game. As soon as people start saying "HL3" they expect some grandiose, ground breaking product.

The story laid out here by Marc is much closer to what you would expect from Episode 3 rather than HL3.