Perhaps most importantly, it doesn’t seem like the Gardens makes any kind of dress code clear on their website.
Disappointed Vacationing Neo-Victorians Demand Their Money Back, Teach Us A Thing Or Two
Ester Bloom

I have no love lost for this couple, they are so self righteous, but I do think the Garden messed up in not having their policy clearly listed on their website. I imagine they probably make money renting the place out, especially for photo sessions. So when two people show up in historical costume (which to most folks just looks like cosplay or costumes) I can definitely understand why the Garden staff would deny admission for what might end up being a guerrilla photo session without their approval.