This is entirely correct. Open sourcing tensorflow was about getting the world to use Google's stuff so that that stuff became more valuable / remained valuable / was possible to hire people with experience in it, not about getting people to work on it for free. The ratio of external to internal contributions is extremely small.

Google saw what happened with other of their technologies like map reduce (not open sourced, the world reinvented in Hadoop and the progress started happening there instead of in MR) and wanted to avoid becoming the bit player in a space they pioneered in. It's a reasonable strategy. (The risk is that you're giving away something that could be a competitive advantage.)

(worked on tensorflow at Google for four years.)