It's actually General Gideon Argus and not "Commander."

When I first read his preview because of the wording of his story I assumed he was a Commander, like Luke as of the events prior to The Empire Strikes Back.

But as I was looking at possible painting strategies for the heroes, I realized his rank on his uniform is that of a General.

A General leading a team of four?

Considering Han Solo had a small strike team on Endor, or Jedi Generals lead units in the Clone Wars , its not completely unheard of in the Star Wars Galaxy.

Possible reason for his promotion .

I guess Gideon was on that transport ship filled with crates of the Imperial Assault boxed game for such a long time he got a promotion at delivery.