Almost all manufacturers say the same thing as Bryston and REL. I mean, think about it: What if they said something like "Using a high quality aftermarket power cord may improve the performance of your new XXXX"
Besides begging the question "Well why didn't you supply a better PC in the first place?" (money, stupid!) Also, recommending you use only the supplied cord limits the manufacturer's liability: If anything happened to your unit while using an aftermarket product recommended by the manufacturer, they would still be responsible for damage or injury (even if the cord wasn't the problem.)
So you have to read between the (legal) lines of such statements.