For most light aircraft the limiting speeds are all indicated, because the limiting factor is dynamic pressure (Q to aerodynamacists). However, flutter isn’t. It’s a factor of TAS, and as TAS varies widely with altitude then if you do go high flutter needs to be factored in. And flutter is not to be trifiled with, for as the great aerodynamacist Theodore von Karman once observed “some fear flutter because they do not understand it – and some fear it because they do.”
As Jim59 explains, his glider's VNE reduces at altitude to compensate for flutter, as will Perlan's. However, to suggest some gliders will simultaneously stall and exceed VNE at altitude is clearly nonsense. The reason why an indicated stall speed at altitude for Jim's glider is not given is simply because irrespective of whether it is at 500ft, 5,000ft or 50,000ft it will still stall at 1g at 37kt IAS.