That is a very unreliable site, for sure much less reliable than this one. The claim is probably due to a simple - but frequent - misunderstanding that also appears to be yours: the mere equation "v+c" is in itself not in contradiction with SR. As a matter of fact, the similar (c-v) is even part of Einstein's 1905 derivation of the Lorentz transformations. And sure, GPS also uses it.
In jargon it is called "closing velocity". For example, assume that a radio wave moves at c in one direction, and a GPS receiver in opposite direction. The relative speed (or closing speed) is then c+v by definition, because the speed of light is c and that of the receiver is v as measured in that reference system.