This is a bit of an underwhelming story.
If Valve actually held a press conference to announce formally that HL3 was in development, that would be news - not because the information provided itself so much, but because they'd made a landmark change to how Valve communicate on upcoming games development.
But as it is, it changes nothing really. Unless Valve were changing to developing *exclusively* "games a service" games, then there's little reason to expect otherwise. Halting Half Life as a series doesn't make sense; there's demand for more.
So what we have is
- an accidental, though not unprecidented "slip of the veil"
- further evidence that "Half Life 3 confirmed!" meme users will wildly speculate on anything tangentally related to the games release
- no change to how Valve communicate (or more accurately do not communicate) on any upcoming Half Life 3 development.
- what was always a very save assumption having more supporting evidence to continue to make it a very save assumption.