I am not a subscriber to WAPO so I am unable to read the article but I did note that it is an opinion piece and says that the DOJ MAY be investigating Trump.

I prefer factual reporting and so far have seen no factual reporting that the DOJ is investigating Trump.

Most reporters have sources in government departments so I think that it is safe to assume that if the DOJ was in fact investigating Trump in any way, that investigation would have been leaked because it is so important. And that leak would have been reported.