From the NLRB's page defining protected concerted action[1] (emphasis mine):
"A single employee may also engage in protected concerted activity if he or she is...trying to induce group action, or seeking to prepare for group action. However, you can lose protection by saying or doing something egregiously offensive or knowingly and maliciously false, or by publicly disparaging your employer's products or services without relating your complaints to any labor controversy."
So, let's say an Amazon employee were to simply tweet "Amazon is shit. Don't buy from them." and got fired for it. Then they couldn't defend themselves by coming back and saying "Well, actually, I was talking about working conditions for Amazon's warehouse employees." Fair enough.
With that in mind, here's what allegedly got them fired:
Emily Cunningham tweeted a thread[2] that begins:
"I'm matching donations up to $500 to support my Amazon warehouse worker colleagues. "The lack of safe and sanitary working conditions" puts them and the public at risk.
It's bad ya'll...".
The thread then goes into detail on that point. It could not be more clearly about working conditions.
Maren Costa tweeted[3]:
"I am matching donations to $500 to support my Amazon warehouse colleagues and their communities, while they struggle to get consistent, sufficient protections and procedures from our employer. DM or comment for match. https://chuffed.org/project/help-amazon-warehouse-workers-ex... # via @Chuffed"
...and that's it. Again, it's very clearly about working conditions.
Amazon doesn't get to make a policy against employees criticizing the way they treat their workers. That's how protections for labor organization work.
[1] https://www.nlrb.gov/about-nlrb/rights-we-protect/the-law/em...
[2] https://twitter.com/emahlee/status/1243441985173651456
[3] https://twitter.com/marencosta/status/1243585580736237568