The management have to find out you're in a union first, then ... why do they shun you, what's the play you're imagining here?
Don't you have employment contracts? People will, at least under rule of law, be treated according to their contracts and your country's employment law??
Does your company currently investigate who you've spoken to about your job and seek to punish you for representing your better interests?
I believe the person you're replying to was talking about how non-union members would be treated by union members. And, at least in the US, the answer is "horrible". The people in the unions are widely known for coming out hard and strong, with considerable bile, against anyone that so much as expresses the opinion that the union might not be the right choice for them. Rats, scabs, etc; pick your insulting name, they're called it.
Don't you have employment contracts? People will, at least under rule of law, be treated according to their contracts and your country's employment law??
Does your company currently investigate who you've spoken to about your job and seek to punish you for representing your better interests?