army of suits

Not army uniforms, but business suits. Army in this context just means a horde, or many of them.

Calling them "suits" in this context intends to portray them as monolithic and impersonal by referencing the "uniform" of the business or institutions.

Another expression that might help you understand how this is being used is the "empty suit" idiom. The expression reduces the person to just a stereotype.

So in this context, the writer is expressing surprise at the casual dress and more personal approach than was expected.