No, they're not really. Synergy horns work on an entirely different set of principles than a tapped horn sub.
please explain why a tapped horn would strictly refer to subwoofers. It seems Danley considers the tapped horn to be a special case of a "multiple entry horn", so strictly speaking you could ding me that I might be referring to multiple entry horns, except that I'm not. Yes, Danley appears to have trademarked the term "Tapped Horn", where it would appear that he considers them a specific case where the positioning of the drivers and use of a ported housing are components of the design, but I don't see where this is strictly for a sub. I also think he's being a little unfair here by marketing a term "Tapped..." when the word for the ports in the cone are "taps" in MEH's so you could say we're talking about "tapped horns" but not "Tapped Horns". Per his Wikipedia page it says this about "Synergy" and "Tapped Horns":
"Danley's Synergy Horn concept was an improvement on the multiple entry design of the SPL-td1. The idea is that multiple drivers in different bandpasses are combined into one horn to make a phase-accurate, constant-directivity single-point-source loudspeaker.
Danley's Tapped Horn concept directs sound from the rear of a horn-loaded driver into ports (taps) that are vented into the horn closer to the mouth of the horn, farther from the driver at the throat of the horn. At frequences such that the wavelength is twice the distance to the taps, the waves routed inside the cabinet to exit through ports in the horn combine positively with direct sound from the front of the driver."
I'm particularly interested in this subject because of Scott Hinson's "MEH" multiple entry horn DIY project and the buzz that exists around that speaker (as well as Danley's Hyperion although those are out of my price range). I will note that from Danley's definition of a TH it seems like Scott's design may be a Synergy Horn but not a TH because the drivers primarily fire into the horn through ports while their back waves that exit through ports in the cabinet are actually on the back of the cabinet and not in the horn. But again, I see nothing about a TH that says it has to be a sub.