Actually these guys were the first US ARMY infantry troops to see action against the Japanese. However even that isn't quite accurate. The US 26th Cavalry was a Philipino Scout unit and it engaged the Japanese as they landed on Luzon. The PS troops were viewed by the War Department (and by the Scouts themselves for that matter) as being a unit of the United States Army. However it was a unit of Filipino troops where were officered by Americans.

On the other hand, Colonel Selleck's force included the US Army's 31st Infantry Regiment (better known as the Thirsty-First or Polar Bears). This unit was the only infantry unit in the USAFFE that was completely made-up of American soldiers. This action was their first action in the Philippines and thus it was the very first US Army infantry formation to make contact with the enemy. Also, I bet it was also the first US infantry formation to make contact with the enemy as well! Yes, the USMC gave the Japanese headaches on Wake Island... BUT, they were members of the USMC's 1st Defense Battalion and thus were actually artillery/coastal artillery and NOT actual infantry! In fact the 1st DB didn't have ANY infantry at all (granted, all USMC troops are trained as infantry). All of its troops were members of the MG, AAA, Coast arty, or members of the 1st DB's HQ/support company (or members of VMF-211).

So this battle really was a first of sorts. Just not the first contact between US and Japanese ground forces.