Your implication that "untested in court" means "likely unenforceable" is quite wrong--unless you see lots of people openly violating a license in cases where it would be economic to pursue them, the more likely explanation for "untested" is simply that the accused infringers get competent legal advice, and comply without wasting their money on a losing court case. Though as other comments note, since the writing of that essay someone (D-Link) finally did refuse to comply without a court order, at which point Harald Welte took them to court and D-Link indeed lost.
And what do you think "not able to prosecute" means? District attorneys (or non-USA equivalents) decide when to prosecute crimes, not private copyright owners. It's very rare for the criminal justice system to intervene in complicated white-collar stuff, especially when a straightforward civil remedy is available. A legal realist might say that means the GPL--and indeed most copyrights beyond those infringed by warez/torrentz sites--is effectively unenforceable criminally, and would in a useful sense be right; but it's enforceable civilly, so no one cares much.