I'm especially impressed by the justification of the Constellation insignia. The only thing left to hash out is to explain the Exeter.
The article does concede that Exeter's insignia was a pre-memo error that wasn't subsequently repeated, post-memo. The meta-reason is all that can be legitimately accepted, barring further information from official sources. To provide an in-universe explanation would be tricky. I can try though!
As a matter of real-world history, the CO's of many ships and bases do have some latitude over the kind of uniform types being worn, along with any deviations from the general Uniform Code (no hat/cover use areas, no salute areas, etc.), but the concepts of insignia devices and military symbolism exist in a different strata of regulation from general uniform use and fore-mentioned allowances. I'm not sure if Starfleet would afford the kind of latitude to Ron Tracy as to allow him to fabricate a custom design for his ship and crew exclusively. Such an action would indicate an almost elitist mentality and provide for some potentially bad feelings between crews within the fleet, non-conducive to preserving inter-vessel morale.
However, it is possible that it is perhaps it is a fleet-sanctioned unit citation/commendation over some action that the Exeter engaged in at some point in her history. Since it does contain the branch designation of the wearer, the insignia can serve a double purpose as a campaign ribbon, indicating special merit in the fleet. There may be other ships who posses this insignia, and we just never saw them because the series didn't last long enough.
That's the best explanation I can extract from deep within my ass. YMMV.
Enterprise's "In A Mirror, Darkly" Defiant insignia, however, stirs up some shit with this, especially since we did see the one TOS Defiant crew member with a standard arrowhead fleet service insignia (which the article quite conspicuously failed to mention).
Other than that one little thing, BEAUTIFUL article!
