The policy prevents employees from speaking, period, if they are identified at all as Amazon employees.
In my view, saying "I wish I were permitted to bring my values to my job doing UI design at Amazon" is not in any way speaking for the company.
What do we think Amazon would/should do if someone external to Amazon went under an entity name such as "Amazon Executive Committee" and posted controversial/activist like content? I would imagine that Amazon would attempt to deal with the negative PR of some people thinking that this is some sort of official Amazon group.