I think the writer is defining "sell" as "give somebody an exclusive right to use it" or "allow people to use it only if they agree to your terms". After all, that's what it usually means to sell proprietary software.

Since any program that contains GPL code must be open-sourced, it's impossible to grant anybody a right to use it at the exclusion of everybody else, or to prevent people from using it against your will.

A very poor choice of wording, though.