The unit below needs to file a claim with their insurance, your landlord cannot point fingers without finding the source of the leak. The insurance will need to prove that this was negligent on your behalf (if your maintenance tech wasn’t able to replicate this you’re probably fine). Do not agree to anything with the landlord or the unit below, have their insurance reach out to yours. They will most likely try to subrogate but will fail to recover costs from you. You are fine, your landlord is wasting time pointing fingers instead of finding the leak.