As an aside, until there is someone willing to go on the record, this story is already longer than it should be.  When the source is the government, "Official Leaker" stories like this should not get the depth or prominence this article got.

………

One of the officials said that even in geographic locations where U.S. intelligence effectively had total ability to monitor the environment for signs of malicious interference, analysts found no evidence of an adversary targeting personnel.

“There was nothing,” the official said. This person added that there was no intelligence that foreign leaders, including in Russia, had any knowledge of or had authorized an attack on U.S. personnel that could explain the symptoms.

Clearly the leakers have official approval to release this information.

Still the suggestions of hostile action were always dubious.

If I were to make a guess, and as my reader(s) know, I always uninformed ones, my guess would be something like an insecticide or fungicide used in transport, or contaminated briefing materials. (Paper manufacturing produces dioxin, for example)