AITA for not choosing my friend in an ultimatum that she made?

I (18f) had a falling-out with my friend group of 13 years because of an ultimatum one of them gave. My friend Summer (19f) has beef with a guy, let’s call him Marcus(18M), who used to be part of our friend group but was kicked out during senior year after his ex-girlfriend spread lies about him to our group. We later learned they were all false. He tried to reach out to us, but others in the group made it nearly impossible, so he gave up trying to communicate with any of us. Even after finding out that Marcus’ ex was lying, none of us reached out, which we know was a bit assholey. Summer and other members of the group created very petty problems with Marcus. My now-roommate, Cass (19F), who was also part of the friend group, and I didn’t continue the beef, but we didn’t stay in contact either. It’s now been over a year since this started, but their beef with him is still going strong. We’ve since started college and learned that Marcus attends the same school and lives in the same dorms as we do. During orientation, seeing as we didn’t have any friends, we ended up talking to Marcus a bit. We also made another friend, Katie (22F), and told her all about the drama. She wanted to hear his side, so we invited him to our dorm to talk to her. It was there that we learned the whole story. Cass and I discussed privately whether we wanted to rebuild our friendship with Marcus. After that, the rest of our friend group found out we were talking to Marcus again, and they got mad at us. This escalated into a fight that lasted a couple of days, and ended with Summer giving us an ultimatum: be friends with her or with Marcus. Cass and I agreed that Summer was being petty, and so to keep the peace, we decided to lie about stopping the friendship with Marcus (which we know is not good). Over the next few months, we heavily distanced ourselves from the friend group due to it being awkward. A couple of days ago, Summer contacted Cass and I to ask why we weren’t talking to them. We told her we felt awkward reaching out after everything, and she said that if we were really friends, we would’ve reached out to her anyway, and that we were killing the friendship. We said we felt unwelcome in the group, and she put us in an uncomfortable position with the ultimatum. She then doubled down, saying she hadn’t forced us to do anything, even though that’s what the ultimatum was. We eventually came clean and told her we had been continuing the friendship with Marcus, even though we had initially said otherwise. We also apologized repeatedly for lying to her, but we weren’t sorry about the friendship we’d built with Marcus. She got really upset with us for lying (valid), but she was more focused on Marcus. We asked her to try to see it from our perspective but she refused and ended the friendship. Marcus and Katie have become some of our best friends at college and even after they told us we weren’t the asshole, we still feel bad about how we handled the situation.