They can consider it patentable all they want, but to patent something (which I have done) you MUST file before you exhibit it publicly. If you exhibit a technology publicly before filing a patent, you lose the right to patent it. What could possibly be more of a public exhibition than releasing an implementation's source code under a public license?
Making available and exhibitions of products
Publicly available products also count as prior art, even though it may be very difficult to determine exactly what the product is made of or how it works. If a device is put on the market before the patent application filed on a feature in that device, the feature is no longer novel. Usually, the sale or other disposal of the product is enough to make all its features prior art for later filed applications. If the product is not sold, but only demonstrated to the public, then only those features which the public could observe count as prior art.