You're making a number of generalizations which aren't accurate.
Whether or not the VHF has regulation is not in question. The microcontroller driving the menu, the screen, backlights, audio amplifier for speaker, beeper for registering button presses the serial interface to external GPS's, the flash memory for storing settings etc. etc....these internal devices all require various regulators to create whatever voltage(s) they each require - probably linear because they're low power but perhaps one or more bucks to knock down the bulk of the voltage more efficiently. The only question, because I don't have the analog/amplifier or VHF radio experience say with certainty is how would the amplifier react.
As for mass market devices well those cheap compact Apple imitation square AC USB chargers use high frequency switching supplies for a few bucks. This dual usb car charger is $5.54 putting out 2A@5V and certainly isn't using a linear regulator which would mean 2A@12V in for a loss of 14+ watts - not in that small package (I have this in my boat). So you're completely underestimating the prevalence and low cost of various regulators, as well as their outright necessity for modern low voltage electronics.
For example something like the victron battery monitor won't have anything that internally runs on 12V. It's all regulated down. This generally applies to other marine instruments. For what it's worth I've designed 50A shunt sensing 12V current measurement circuits.
Again, we're largely wasting everyone's time with this debate except for the small area where this stuff intersects debugging marine equipment. The point to what I'm saying is that with modern digital based equipment, input voltage issues are farther down on the priority list of debug than they might have been in the past. Not to mention the fact you keep glossing over which is that all these radios are specified to operate between 10V and 15V - if 10+V is getting to the VHF when transmitting at full power that's in spec (if and only if it's 10+ volts WHILE transmitting and drawing full power). The next logical debug step is to look elsewhere. Of course starting the engine to raise the input voltage is low hanging fruit and I don't discourage it.
The fact that two entirely different radios measure the same (how close was it actually?) output power calls the tester in question more than anything else. It's kind of like measuring two cars maxing out at 35Mph with the same speed gun and concluding the gas is bad. Two different radios would have significantly different reactions to low voltage/high resistance input power which makes seeing the same output power pretty unlikely.
Edit: This picture actually shows the IC and inductor for the internal 10W buck conveter (DC-DC) of this $5 usb car charger.