I already know about the National Labor Relations Act (NLRA) of 1935 that basically prohibits companies from saying you are not allowed to talk about salary with coworkers, but my situation may not be covered by this act.
Company A: I do not work for directly and also the company I applied at.
Company B: I work for directly in support of Company A.
I have worked at company B for a while now and recent applied for a better position at company A and received a job offer. In my excitement I went to speak with a couple friends about the offer and this included the salary I was offered as well. I didn't think this was a big deal however I was told that someone from company A overheard my conversation and reported me to their HR department about it and was then told I put the job offer in jeopardy by speaking about my salary.
I am aware that the NLRA protects my right to talk about wages/salary but the NLRA does not apply to everyone.
My question is this:
If I work for company B and speak to someone from company B or company A is there a situation where NLRA does not protect my right to speak about salary?
Technically I do not work for company A yet so can they rescind their offer letter?
Note: I am not seeking legal advice just clarification on the NLRA wording.