So even if they don't know what you're saying on Signal, they know who used it at what times.
Plus Signal groups are unwieldy with many participants, plus it's just a PITA to type on your phone vs a keyboard.
Also, does GOOG MITM employee connections? Many large banks and hospitals make you install a root cert to use the wifi.
In short, there are many nudges pushing employees to use "insecure" organizing methods.
Also from a legal perspective, if you have a history of allowing internal debate, you open yourself up to liability if you silence that debate when it touches on the workplace.
(It's protected speech in the states to discuss working conditions, and it can be considered retaliation to not allow discussing conditions if other non work talk is allowed)
And a large part of organizing is bringing people into the fold, so you'd still need to be organizing at work somehow, even if all you're doing is asking for external communications methods to continue the conversation. There's a much higher drop rate that way though.
Legal protections are not very helpful in a context where the other party has unlimited legal resources and time is not on your side.