There's a big difference between "TensorFlow is great, and Google Cloud is great, and you can bet your butt that TensorFlow will run well on GCE" than "Run TF and get locked in to GCE". In fact, as I read it, the selling point is actually much closer to:

"TensorFlow is great, GCE is great, the two work great together, and it's open source so you can trust that you _won't_ get locked in." In other words, no-lockin is a deliberate part of the value proposition of TF+GCE.