You'd think, but the reality is most companies and researchers do not release vulnerability findings like this to the pubic without giving those affected by it a chance to research it themselves. Just throwing out to the public willy-nilly would be an act of gross irresponsibility. So yes, companies like Microsoft, Apple, Google, Intel, AMD, Nvidia, etc., etc. will keep such info confidentual until they have time to solve the problem. Intel and the researchers were being responsible, not secretive or sneaky. Meltdown is effectively solved and that solution will be refined in the coming months. And this is why we have a fix for within days instead of weeks or months.
This.
You all have to keep in mind that if someone finds a leak in your CPU architecture, there is no realistic way to adjust that on a hardware design level anyway, any fix like that is one or two years ahead of us at best. The fact they found this in June, only months before CFLs release, is proof of that in itself. Yes, they knew it was in there, and yes, they were already testing and finding fixes for Meltdown back then. I think its safe to say that we won't see a hardware adjustment until Ice Lake, or beyond.
Communicating leaks before you have solutions is possibly much worse than announcing them days prior to a fix. The entire industry works with that premise, its really telling that people here think otherwise - its a clear sign you have no clue of how this industry functions. While not the best layer of security, Security by Obscurity still is a layer of defense, and it was utilized here.
On the other side of the fence, even AMD releases their CPUs with knowledge of Spectre's existence, and even after official announcements were to be found on Intel's website, AMD's website did not contain a SINGLE TRACE of Spectre's existence. This is a strategy, too, and it shows in everything AMD has put out regarding this issue: they want to silence the issue ASAP, they are making it 'small and inconsequential' if you read their PR. I'll leave it up to each individual to decide what's better...
The bottom line remains: both Intel and AMD had this knowledge around the same time, and the decision to keep this quiet until now has been a unanimous one across ALL related companies. Any alternative decision is much more damaging: to end users, to the industry, to the overall level of trust in every PC we use, and all of the data we handle.