> If GPL licensed source code is required for a program to compile, the program must also be under the GPL.

This is obviously wrong too, or else everything compiled by gcc would need to be licensed as GPL. If you distribute a GPL licensed program then the recipient must be able to build the equivalent program from source (e.g. you can’t rely on an internal toolchain that doesn’t have a publicly available alternative) but that isn’t what the author is saying.

No, this is correct. You cannot sell it, because you do not own it. What you can do is distribute it, which is another matter. The author is right.