I don't think you're interpreting that section quite right. That section is talking about whether or not Bazel is fully _developed_ in the open, and the answer is "Unfortunately not".
What they mean is that changes to the internal source of Blaze often involve changes to both the open sourced part, which is Bazel, and the closed parts, which are additional rules that are neither open sourced, nor included in Bazel (Blaze has about 5x as many rules as Bazel).
It's best to make atomic changes, so rather than split the changes, review and submit the open source changes externally, and the closed rules changes internally (which would complicate reviews, testing, syncing and rollbacks), then pull in the external changes, they submit these cross-code-base changes internally, then dump the change into the external repo. The next paragraph on that page makes it clear that the code is open, even if not all of the development process is.
http://bazel.io/docs/build-ref.html#rules
For example, cc_binary is a rule.
Rules are the things that know how to take whatever is specified as it's inputs, do something to them, then produce some specified set of outputs
Google has a large number of rules (IE far far more than just the rules you see in bazel).
As part of open sourcing, they have stared out by open sourcing about 10% of those rules.
Some of this is because they are google-entangled. Some of them don't make sense to the open source community.
etc
What they mean is that changes to the internal source of Blaze often involve changes to both the open sourced part, which is Bazel, and the closed parts, which are additional rules that are neither open sourced, nor included in Bazel (Blaze has about 5x as many rules as Bazel).
It's best to make atomic changes, so rather than split the changes, review and submit the open source changes externally, and the closed rules changes internally (which would complicate reviews, testing, syncing and rollbacks), then pull in the external changes, they submit these cross-code-base changes internally, then dump the change into the external repo. The next paragraph on that page makes it clear that the code is open, even if not all of the development process is.
To be clear, all of Bazel is open source and the source is available here: https://github.com/google/bazel