Ive been under the weather recently and kind of fuzzed up with drugs, so I just remembered why that particular comment was bothering me. Thats actually a pet peeve of mine with how TVG is often talked about and marketed.
Yes, TVG can compensate for deeper returns - BUT - there are downsides to it and thats not really its main purpose.
The main downsides of turning up the TVG is that it actually reduces the over all Gain level and it can completely hide shallow returns that you would other wise see if TVG was lower.
TVG's main function is actually to reduce noise. TVG is listed in the manual under the Noise Rejection heading. From the Garmin manual:
More importantly from my perspective, is that claiming the TVG makes a 10 lb fish look the same at any depth is really not true UNLESS that fish is in exactly the same place within the sonar cone at every depth. The odds of that are next to zero - especially at greater depths and wider cone angles.
Remember my posts from earlier about how equal sized fish at the exact same depth can have very different returns and very different sized arches? This goes back to the fact that a sonar cone's signal strength varies depending on where the target is within the cone. The outer edge of the sonar cone could easily be as much as ten times weaker than the center of the cone and still create a fish arch on the screen. That means a 10 lb fish on the outer edge of the cone could have the same exact return/arch as a 1 pound fish in the center of the cone.
Your sonar has no way to tell you if that fish/return is in the center of the cone or on the outer edge or anywhere in between. TVG doesnt change that.
So, at best, TVG can maybe, sort of, possibly, if your lucky, kind of equalize the arches at different depths - but the odds are against you and you really have no way to tell if its doing that or not.