Why are people so certain Half-Life 2: Episode 3 has been cancelled?

Marc's leaks doesn't even imply that. Yes, I know that likelihood of ever seeing it is low. Yes, I know how Valve and Gabe have changed. But it still doesn't confirm that Episode 3 has been cancelled. In my personal point of view, at this point there's nothing but uncertainty. Valve has no idea what do or where to go with it, but instead of completely discarding it, they left it there in hopes of one day returning to it.

In my opinion, the only reason Marc shared the potential story line is because he is someone extremely passionate about his work and wants to share it with people. And considering that Episode 3 is currently in development hell and that it's uncertain whether it will get out of it or not, he decided to share a vague idea of what could've been to satisfy the fans.