Liable for broker fee ?

This is not intended to be a legal opinion, but it strikes me that the issue of any payment to the broker here is between the leasing office and the broker; was there any contract for the broker to lease the apartment? Did that contract give the broker an exclusive? Either way, my personal (not legal) opinion is that the tenant applicants owe nothing to the broker. If the broker did not have an exclusive listing agreement on the apartment, then the leasing office simply acted as a competing broker who closed the deal where the initial broker had failed. It strikes me as analogous to a homeowner who lists his apartment both FSBO and through a nonexclusive listing agreement with a real estate agent. The broker could fight with the owner over whether a deal such as the one described here is really FSBO or whether the broker generated the deal, but either way, the dispute is between the broker and the owner, not the broker and the buyer. Again, this is not intended to be a legal opinion. The question seeks legal advice that can only be provided by a qualified attorney (not me), so I would ask a real estate attorney if you want the question answered properly (or a court if you want it answered definitively).

As a broker, it does not sound like the tenants would be liable to pay the fee. If the broker has no DOS disclosure and no signed fee agreement, he/she has no claim to a fee.

You are a lawyer and you are giving your opinion, that's a legal opinion unless your caveat is meant to say that you are actually stating the opposite of what you would correctly say.

If someone complains of chest pains to a doctor, the doctor says you are having a heart attack but that isn't my medical opinion, what should the person be thinking, that it's not a heart attack?