> NYTimes states they are aware of the identity of the person so it's not like the validity of the article is suspect.
That requires to place 100% trust in NYT, which you may be willing to do, but it's not a law of nature. Journalists in search of a scoop have been known to confuse what they want with what really is (remember the sordid Bush memo affair that ruined Dan Rather's career?) and even if not intentionally deceitful, being easier to deceive if the information confirms their biases.
> For the sake of discussion, what ethical arguments do you think there are for finding out who wrote this?
Knowing who wrote this allows to adequately evaluate whether what is said there is true, what vested interests or grudges may the author have and what information may they have from the first hand, second hand and what information they couldn't have and if they claim they have it they must be lying. E.g. if somebody who doesn't have clearance says they have been on classified briefing and heard Trump say so and so, it would be clear they are either lying or relaying third-party information as first party.
While for the author of the article discovery would be a negative, for a voter that wants to be informed, and wants to independently evaluate the worth of the information knowing the source may be very useful.
Also, since this article does not seem to be disclosing any classified information, the worst thing that could happen is that person would be fired. People got fired from Trump administration many times, some after a very short time, and none of them suffered any heavy consequences that would make any act leading to such outcome unethical, such as death, heavy bodily or mental harm, full financial ruin, etc.
So it appears to be clearly within ethical bounds to research this question, no less than any question that any politician would like to hide to promote their goals. While each such revelation inevitably would hurt somebody's interests (otherwise nobody would try to hide it!), there's nothing unethical in still trying to reveal it.