Steam can actually be HOTTER than boiling water so I don't doubt it will sanitize. But it could also release chemicals from the plastic that boiling water never would, because once again, steam can be hotter than boiling water. I doubt that'll happen, but theoretically it could be subjecting the plastic to temperatures much higher than it was rated for, having the exact opposite effect he is looking for, which is to sanitize without exposing himself to "toxic" sanitizers.
Just to be clear, steam isn't hotter than boiling water; it has more energy (latent heat). If that fermentor is rated past 212F, it should be fine with steam at atmospheric pressure.