As you found, individual tolerance can vary widely.  I typically wear a
full 3mm and particularly if its a low-activity level dive will still
get cold after an hour or so, so I've been contemplating adding a
warmwater hood or "beanie" for 80F dives.  Generally speaking, I won't
strip down to just a dive skin until its above 84F or so.

You're right that a wetsuit is simply a form of insulation.  This is  
very basic "advertising copy" abuse of the English language.

No, you're right...you just forgot a little physics en route.  

The advertising copy isn't technically a lie because every warm body
always produces heat in Radiation, Conduction and Convection forms.

The advertising copy is, however, has been deceptive as to communicating
the level of significance of its claim.  

For radiation transmission, you might recall the term "black body
radiation" being used with it.  Generally speaking, the hotter the body,
the more radiation transmission there it puts out.   What this means for
our human scuba diver is that since his skin temperature is ~85F (and
not 3,000F) the amount of radiation he's transmitting is very small.

And that's just in air.  Once we dip that diver in water, his conduction
and convection heat transfer rates soar in comparison to air by more
than a full order of magnitude...IIRC, around 25x greater.

So even though Physics says that there will be some radiation that some
percentage of it could be reflected back, from a "percentage of total
sources of heat loss", it is so trivial that it may as well be literally
zero.  

In other words, the Ad's stating the literal truth, but neglected to say
that its scientific contribution is quantitatively insignificant.  IMO,
this makes it a deceptive and unethical advertisement.

FWIW, what this also happens to mean is that any perceived warmth gained
by this "feature" is really going to be more a psychologically-derived
"Placebo" Effect in those people who believe the BS marketing hype.

-hh

While the "cold" (which doesn't really exist, anyway) can't penetrate the water can,
and besides insulating from conductive heat loss wetsuits reduce the flow of water
against your skin. That's why wetsuits are supposed to be a bit tight. I don't know
exactly how the ad was phrased, but it may not have been misleading so much as poorly
written. Of course advertising is also targeted to the LCD, so if they got it all
right it wouldn't be as efective.

Titanium is one of the new buzzwords in the world of marketing. Georgia pacific is
marketing plywood with the tradname "Plytanium".

You could be wrong, but it's really difficult to be overly critical of advertising.

As for wetsuit decisions, here's my two cents worth. As with any other insulating
clothing layering gives you versatility. Instead of a 5mm shorty I'd opt for a 3mm
farmer john and a 3mm shorty. If you don't get chilled easily or the water is fairly
warm a polartec diveskin has the advantage of neutral buoyancy. My usual warm water
outfit is a 2mm farmer john shorty under a polartec skin, and a polartec hood. I
started wearing the hood because the mask strap bothered my ears, but it helps a lot
with warmth.