5.c says "This License gives no permission to license the work in any other way, but it does not invalidate such permission if you have separately received it."

5.c says "This License gives no permission to license the work in any other way, but it does not invalidate such permission if you have separately received it." So you can distribute a BSD project with readline, and recipients are free to choose the BSD license for the BSD part and then distribute a closed source binary linked against a compatible readline replacement. The GPL does not invalidate the permissions granted by the BSD license. The only goal of the FSF here is that the source code for anything distributed linked with readline be made available under the GPL. They don't stipulate that it only be made available under the GPL and not any other license.