Interesting point, but I think it's meant to be as it's written in plain language. Work-related to a "reasonable" person would mean anything that is directly needed for workers ability to perform the job. Since the company was working without a union, it's self-evident that conversations about unions are not needed for workers to perform their job.
The same goes for fights with children or spouses, having trouble paying bills, religious and political beliefs, losses by favorite sports teams, current fashion, current music, golf, etc. Each might affect a worker, perhaps extensively, but are not work-related. The same goes for a union that doesn't yet exist at that site.
The law I think means, if people are banned from raising their personal issues and beliefs at the workplace, then all conversations regarding non-work related subjects can be banned, therefore, conversations and information about unions (that don't exist at that site), can be banned.
Is that realistic? Sure. I've worked a number of jobs where there was no time or opportunity to have personal conversations. I've worked others where we were warned about numerous topics that should be avoided at work, e.g., politics and religion. Is it common? Perhaps not. But the law makes it clear, imo, that there is no legal right to discuss unions at work unless the employer has granted that right, implicitly (by not attempting to suppress non work-related speech inaction) or explicitly.
