Sanderxpander
He's talking about foldback distortion I think, which can compromise high frequencies in some very specific situations. Ampsims and some particularly bright VSTis are prone to this, if I remember correctly from Craig Anderton's testing. I haven't personally had any problems but I believe there is a control bar icon somewhere on the right hand side (probably labeled "2x" or something) that enables upsampling.
I remember reading that. It's likely why some VST's sound brighter to me at higher sampling rates. My understanding, and m y own experience, tells me that the difference is quite subtle, especially in a mix.
Regards,
Dan