They're also spreading this nonsense at my hospital where we're actively organizing right now. It preys on people's ignorance of how legal contracts work.
Your lease doesn't guarantee that your landlord won't violate its terms either. But it does give you the ammunition to take them to court and win compensation, if they do.
The same applies to a union contact. It's legally binding in the same way. Sure the company technically can violate the terms of that agreement. But the union is going to escalate the issue to the NLRB and/or sue them in court, if they do. And you will win, if you provide the evidence that they violated the legally binding contract.
This is what disingenuous corporations call "not guaranteed"
I think what they're trying to say is that being in a union, by itself doesn't guarantee higher wages or better benefits. Which is true, but incredibly misleading since you sure as hell aren't getting those wage/benefit increases without union membership.
Unions literally cannot guarantee you money, dude. Other than what's in their coffers, and what is "drawn up" in advance, collected for, and spelled out in a written agreement.
Especially fragile new unions, they don't have jack shit going for them. The company isn't going to help it form, and they might choose to fire dissenters attempting to unionize. During a strike or demonstrations, even with a formed union, the company isn't going to pay the workers for not working. In this way, the company is correct in saying "there are no guarantees of pay". Hell if they don't like you there's no guarantee of a job at all, if there's no established union.
It's all talk, speculation until a formed union draws up a legal, enforceable contract that both company & union agree to. CBA, as it's called (Collective Bargaining Agreement).