It appears you're confusing Bigtable with Datastore (to be fair, Datastore is built on Megastore, which is built on Bigtable, so it's an understandable confusion), but let's be clear: Google Cloud Bigtable != Google Cloud Datastore.
Sorry about that, I had difficulty finding anything about the consistency model of BigTable. Upon further reading on SSTables and how BigTable distributes them across nodes, it appears there is full consistency but if a node goes down the data on it is inaccessible?
If a node goes down, another node will replace it quickly. The node (aka tablet server) doesn't own any data. The data is stored on lower level storage layer.
It appears you're confusing Bigtable with Datastore (to be fair, Datastore is built on Megastore, which is built on Bigtable, so it's an understandable confusion), but let's be clear: Google Cloud Bigtable != Google Cloud Datastore.
The URL you cited about consistency models: https://cloud.google.com/datastore/docs/articles/balancing-s... is entirely about Datastore, but you referred to it as Bigtable.