Job requires joining of labor union

So I had a job interview over the phone yesterday that seemed to go well, however it’s part-time and they also disclosed in the interview that one of the conditions of accepting the job would be to join their labor union (wasn’t mentioned in the job post). I don’t want to join a Union. I read on the Unemployment site that you are allowed to refuse a job offer if the job requires you to join or resign from a union but does anyone have experience refusing a job offer because of this reason?

I haven’t gotten a call back from the job but if I end up having to decline, what to do I do? Do I need to disclose that to unemployment? How would I even do that?

Thanks.

Edit: for clarifications