First date didn’t disclose status til after the fact, is this normal?

Context here is important! I (22F) am completely monogamous and downloaded a dating app for casual reasons. I am planning to move states in six months but I’m still interested in building a more casual romantic connection with someone. My dating profile is set to “still figuring it out.” Anyways, I met this person (20&nonbinary), I thought that I had super hit it off with them and we had a great first date. They had “short term fun” in their bio and explained that they were also moving this year. Cool! After the date, they texted me to disclose they were in a poly relationship. I’m uncomfortable with that but still open to being friends because we did connect. I just feel weird about talking to someone in a committed relationship (because again I don’t understand it, I’m monogamous, fundamentally not compatible) As I’m not poly, is that a red flag??? Is friendship gonna be safe with this person or is it a big no no to not disclose until after the first date? Thanks everyone! Edit: Thank you guys so much for the feedback! The responses to this post were (mostly) overwhelmingly positive and I’m greatful for that. I am going to state that I am seeking a short term connection with the possibility of exclusivity if it’s the right person. I don’t know how much luck I’ll have but we’ll see. I guess you could call me a “short-term serial monogamist.” This forum has helped me confront my judgements about polyamory and the more I absorb each comment, I’m even a little interested! I love the loving nature and open communication you guys practice. I don’t think it’ll ever be my lifestyle but my initial inherent beliefs that polyamorous people are unfaithful or selfish have been dispelled. I’m sorry to the community for any harm I’ve caused by feeding into the stigma. Y’all are cool ass people. <3