Competitive advantage is protected by custom hardware (and huge proprietary datasets).
Everything else can be shared. In fact it is now advantageous to share as much as you can, the bottleneck is a number of people who know how to use new tech.
"Sun's two strategies are (a) make software a commodity by promoting and developing free software (Star Office, Linux, Apache, Gnome, etc), and (b) make hardware a commodity by promoting Java, with its bytecode architecture and WORA. OK, Sun, pop quiz: when the music stops, where are you going to sit down? Without proprietary advantages in hardware or software, you're going to have to take the commodity price, which barely covers the cost of cheap factories in Guadalajara, not your cushy offices in Silicon Valley."
Developers are a complement to hardware, software, and data based businesses. Google probably spends more on employees than on hardware or data. They really want to commoditize developers that can work on their stuff.
Google's business is based on having more and better data than their competitors. In many cases they have been happy to commoditize hardware, making open their datacenter designs for instance. That helps to cheapen their costs for building datcenters.
In other cases they open source software, like tensorflow and go language. These choices are made to commoditize developers. Google wants there to be a big pool of people who know how to use the technologies that Google uses. More developers means less costs for Google to hire and train employees. Which is their biggest expense: win!
With the TPU, as long as no one else is doing that kind of hardware for machine learning, it is a proprietary advantage to keep it secret. But at some point the logic flips: when others start to do similar things Google would rather commoditize their version of the tech. Because the lesson of the last 40 years is any widely used hardware WILL become commoditized. The inertia is with software codebases and developer knowledge.
Developers developers developers developer developers!