Well, Apple only fail to release a free runtime environment for Yellow Box when the project was killed. Without that runtime environment, applications written for Yellow Box for Windows wouldn't run.
Apple had promised (originally) that the runtime environment would be made available for free or could be included with developer's applications. That was the part that Apple backed out of when ending Yellow Box for Windows.
Anyone wanting to play with Yellow Box for Windows and Yellow Box applications need only find an old copy of WebObjects 4.x for Windows. Everything needed is there.
As for the validity of this e-mail, it seems quite questionable to me. This passage:
"As you probably know it, the Yellow Box for Windows was NeXT's project of porting Project Builder (known as Xcode today) and the complete NeXT API (known as Cocoa today) to Windows, allowing developers to create a Windows binary by simply ticking a check box."
makes me wonder about the writer's understanding of Yellow Box.
First, Yellow Box was Apple's name for the APIs used in Yellow Box for Windows, Rhapsody/Mac OS X Server 1.x and WebObjects 4.x. It was based on NeXT's OpenStep APIs, but had a number of changes. NeXT didn't exist anymore when the term Yellow Box was first introduced.
The previous runtime environment before Yellow Box for Windows was OpenStep Enterprise for Windows.
That was a NeXT project.
Secondly, the "simply ticking a check box" thing hardly worked in NEXTSTEP/OPENSTEP/Rhapsody when dealing with different hardware platforms, it was absolutely not as simple as clicking a check box to make Windows apps. Those required a lot of extra work including a ton of time in Interface Builder.
Now, that having been said, the question of Apple doing this is still an interesting one. The main reason that I saw for Apple dropping Yellow Box for Windows was that it provided too many of the underlying features of WebObjects which would have cut into sales.
Of course WebObjects isn't the revenue earner these days that it was back in 1998/1999, so the idea that Apple would go forward on this isn't that far fetched.
... I just don't see this e-mail as having any link to Apple in anyway.
As for wanting an open source version of Cocoa, the GNUstep project has been attempting to keep the OpenStep Specifications as up-to-date (as close to Cocoa) as possible. It seems like most of their time has been spent on trying to recreate the NEXTSTEP/OPENSTEP environment rather than making a development platform. The last time I checked, it didn't seem like there was a lot of apps for GNUstep... specially compared to the number of apps i have installed (or can install) on my OPENSTEP system.