There is at least fault with presenting all that assertion and speculation as "Law enforcement absolutely has that capability".
The first bit that approaches being evidence is the CNet article linked from the link, and it says
Details of how the Nextel bugs worked are sketchy. Court documents, including an affidavit (p1) and (p2) prepared by Assistant U.S. Attorney Jonathan Kolodner in September 2003, refer to them as a "listening device placed in the cellular telephone." That phrase could refer to software or hardware.
So the absolute proof is a court document that doesn't provide any details about the mechanism used to compromise a dumb phone in 2003.
The first bit that approaches being evidence is the CNet article linked from the link, and it says
Details of how the Nextel bugs worked are sketchy. Court documents, including an affidavit (p1) and (p2) prepared by Assistant U.S. Attorney Jonathan Kolodner in September 2003, refer to them as a "listening device placed in the cellular telephone." That phrase could refer to software or hardware.
So the absolute proof is a court document that doesn't provide any details about the mechanism used to compromise a dumb phone in 2003.