They did not, that is (perhaps implicitly) part of the problem description. It never says the honest miner finds block N, it asks when they are expected to impl

They did not, that is (perhaps implicitly) part of the problem description. It never says the honest miner finds block N, it asks when they are expected to implying it hadn't happen up to that point. Also, the dishonest miner found the "next block" but did not broadcast it at t=0, which is essentially irrelevant information because it doesn't matter whether the honest miners know about the hidden block or not, either way it will take them an EV of 15 minutes at any given point in time regardless of knowledge