It depends on which protections you're talking about (note: IANAL either). From what I can gather, it's true that the organizers may not be covered under laws protecting employees who report concerns internally or file EEOC or OSHA complaints. However, any activities related to collective bargaining (which may absolutely include organizing and participating in protests) are legally protected under the National Labor Relations Act, and would not be affected by "contractual duties" or "utilizing good faith channels for whistleblowing."
Whether the organizers were covered by the NLRA is another matter; those with direct reports are likely excluded by the 'supervisors' exemption, which has been expanded over the years to cover pretty much anyone in a managerial role. For those who are covered, Google's actions could absolutely meet the legal definition of retaliation.
As an aside, the legal standards for what constitutes 'retaliation' are not the only ones that matter. They may manage to avoid a lawsuit, but they will suffer harm to their reputation regardless. I, for one, could not care less whether the law allows them to retaliate against the protest organizers. To me, this is just one in a long line of reasons why I scratched Google off my "short list" of potential employers.