Nah, he's probably confusing things here.

First, the Boshin war was a Japanese affair. When the British asked Saigo Takamori if Satsuma and Choshu wanted help from Britain, Saigo turned the offer down. The Tokugawa shoguante could have called in the help of the French (as France was a major backer of the shogunate), but they too chose to settle things on their own as well.

So on 18 Feb 1868, Western powers declared neutrality in this civil war.

Once Satsuma and Choshu forces controlled Edo, and the pro-shogunate lords in northeastern Japan lost, the Meiji government became the legitimate government.

The major U.S. involvement in the war came after this point. In early 1869 the U.S. government sold the former Confederate ship, CSS Stonewall (an ironclad ship) to the new Meiji government and it played a role in defeating the former shogunal forces holding out in Hokkaido.

On a separate topic, the US had a big presence in Hokkaido's Frontier Agency (Kaitakushi, 1869-1882). William Smith Clark of Sapporo Agricultural College (today Hokkaido Univ) is perhaps the most famous American in Hokkaido from this period.