Being one inclined to believe that the Giffords do in fact know the difference between a dime and a pre-zinc copper penny, I'm inclined to believe that a Toltec 100 could in fact distinguish between the two just the same as Tesoros of today can.
However, I think there's a slight misconception here: No detector will tell you the difference between a copper cent and a dime in and of those coins themselves. Just like why, for example, a detector will give you the exact same reading on a dime and an old zinc canning lid. It matters not a bit to a detector what the thing(s) actually *is* -- the only job it was built to do was indicate to you the relative conductivity of whatever ends up under your coil. If it happens that what turned out to be a copper cent somehow gave you the same reading/signal as a clad dime (for example, because of masking or bleed-over from an adjacent target you're not readily aware of, heavy spot mineralization or a small hot-rock "clinker" around or below the target, etc.), you can't really blame the detector as somehow being inaccurate. It's just working within the laws of science and physics that mfrs have accepted as true and build their circuitry accordingly.
Now, the day someone comes up with a detector that can tell you with any reliability whatsoever that you've got a dime instead of an old zinc canning lid or a big hunk of iron with the same relative conductivity is the day every single detector made to this day will become obsolete overnight.
Scott