There is a distinct difference between wearing BDUs/ACUs with no insignia and wearing them with.
It is not uncommon for some people to don these garments, especially when they are either downrange or in the presence of military personell but this is not to be seen as wearing the uniform of the US military.
It is interesting that a former Soldier commented that she wore a uniform. It is also possible that Mr. Foer made a simple mistake and mistook being told she was wearing BDUs and translated that directly into 'uniform'. It is also possible that she had a clearance. They do give them to civilians.