Uniform is a uniform is a uniform. It would be a costume if it was a bad attempt to "look" like a uniform, but the BOB ones were not.
From Merriam Webster (italics by me):
Costume: an outfit worn to create the appearance characteristic of a particular period, person, place, or thing <Halloween costumes>
Uniform: dress of a distinctive design or fashion worn by members of a particular group and serving as a means of identification; broadly : distinctive or characteristic clothing
There costumes,get over it.
Unless there relocated "UNIFORMS" from that period of time,which of course they are not.Therefore their costumes.
They're not costumes unless they're an inaccurate portrayal of a uniform. For instance, a child's soldier outfit for halloween qualifies as a costume, as it is simply an alglamation of an idea meant to represent a generic outfit. Meanwhile, the clothing worn in those movies, or by reenactors, are uniforms proper, as they are exacting reproductions of the uniforms. Saying that they are not is like saying that a modern day reproduction of a frock coat suit of the 1880s is not a suit.