Out of curiosity, which ad campaign are you talking about?
As for which OS (Win/Mac/nix) controls the majority share of developer desktops, I feel like it's always going to depend on what you're developing, so talking about the overall "biggest slice of the pie" for developers is less meaningful than talking about who has the biggest slice in the consumer space.
For example, a backend web developer might look at this "Winbuntu" thing and suddenly be attracted to the idea that they could trade their Mac in for a PC that lets them do all the UNIXy stuff they need for their job, but at the end of the day lets them play the latest PC games...
...unless SteamOS continues to grow in popularity, in which case Microsoft loses share because a Linux-based laptop suddenly seems like the best choice for a gamer-developer.
On the other hand, if we're talking about a company handing work laptops out to employees, frontend developer-designers are likely to continue preferring (requiring, really) Macs for a long time to come, and that likely means that it makes more sense to keep a common platform and hand Macs out to everyone, since so many server devs are already well-accustomed to using Macs. And though Windows might eventually become attractive enough to professional designers, Linux is deeply neglected in the design-oriented space.
But that's all just web development, which has much more fluidity than other types of development. Game developers will continue to develop on the platforms that they intend to support (or Windows for consoles, at least for the time being). iOS developers will continue to develop on Macs. Mac developers will develop on Macs, Windows developers will develop on Windows, and Linux developers will develop on Linux. I'm barely an Android developer, but it seems to be slightly more natural to work on a Mac or Linux machine, and yet "Winbuntu" would likely remove that advantage.
I agree that with Windows embracing Linux so deeply like this, it certainly opens the door for a lot of people to make the switch-- personally, I bought a Surface Book because I was excited by the hardware, but quickly returned it once I realized how unhappy I was without native access to a terminal. If Ubuntu continues to flourish as a fully-fledged aspect of Windows, I might consider buying the Surface Book 2.
But my personal anecdote also illustrates the greater point-- this opens the door, but it doesn't push anyone through it. I was tempted away from Apple because they've stopped innovating on their laptops. In order for developers to switch to Windows, they'll have to be tempted for their own reasons. And old habits do die hard.