Perhaps the hardest thing we do as human beings is something we think we do all the time: communicate. One of the fundamentals of communication is that we have a common language. In this case, that may be at the root of some confusion here. "ABS" and "skidding" aren't necessarily exclusive concepts.
First, ABS is the acronym for "anti-lock brake system." In a nutshell, ABS works by monitoring wheel speed and when the system sees the wheel speed go to zero (0) - where the wheel is locked - the ABS system releases the wheel and then reapplies braking to the point where again the wheel is locked and slipping relative to the road, then releases it again and it does that over and over, for some systems, as much as 15 times per second. Yes, there is a period where the wheel(s) are locked but it's for a very short period and the cycle rate is so high it's not something easily seen by the naked eye. The underlying concept behind ABS effectiveness is that a wheel at "threshold" - the point right before it fully locks and slips and the wheel speed goes to 'zero" - is slowing the vehicle more effectively than one where the wheel speed is zero and the tire is sliding on the road in full slip (the wheel speed stays at zero).
Here's where the user's intent or meaning of a word is important: we should be talking here about a difference between "slipping" or "skidding" relative to the road and "effective threshold braking."
So, what's a "skid?" In your post, you wrote:
... If you try heavy breaking on a gravel road you will see the results and the fact that they do indeed skid...
What you might be referring to as a "skid" others in reconstruction might be more inclined to call "furrowing" in the gravel road example. Normally, a "skid (mark)" is a mark left by a locked wheel. We see the results of non-ABS equipped cars braking and skidding on pavement in the form of "tire marks" which are generally a combination of molten rubber from a locked and sliding tire combined, in some cases, with roadway materials like tar or oils resulting in what we can call "skid marks" or "tire friction marks."
In your example, on a gravel road, the ABS equipped braking vehicle is locking and releasing the tire(S) so fast that the tires are essentially pushing dirt along as the vehicle slows toward a stop. The "marks" aren't "skid" marks in the traditional sense (like you might find on a paved road from non-ABS equipped vehicles) they're simply a displacement of the gravel as the tires are being slowed and released then slowed again. But the furrowing doesn't "prove" an ABS system allows the wheels to lock up during braking.
Now, in terms of what you might think of as "skid marks" on a paved road from ABS equipped vehicles, no, ABS equipped vehicles don't leave "traditional" locked wheel marks and that may be part of the source of some of the confusion here: the difference between the appearance of a non-ABS equipped vehicle's tire marks and those left by a vehicle with ABS. In heavy, ABS-assisted braking, an ABS equipped vehicle will leave faint, short lived marks on the pavement which - depending on the type of pavement among other things - might be relatively harder to see on some roads.
At the end of the day, ABS equipped vehicles do leave "tire marks" during heavy braking but they don't have the same appearance characteristics as marks left by non-ABS equipped vehicles. A properly functioning ABS equipped vehicle will lock and release the tires during heavy braking and that effects the appearance of the marks these wheels leave. But, and maybe this goes to the heart of the question, the appearance of a "tire mark" (on the pavement or on the road) isn't, by itself, an indication of tires being locked or not.
Going back to your gravel road example, try this: Drive down a DIRT road over a part of the road with other tire marks. Look at the marks left by your car. You'll see the tread pattern left by your tires. Now drive back through the same area but this time, go into full on ABS-assisted braking. Now look at the characteristics of those marks. Note that the tread pattern's no longer distinguishable as it was in the case of the rolling wheel, no braking drive through. If you can, find a non-ABS equipped vehicle and do the same hard braking and compare the characteristics - on the dirt road - of the ABS-assisted and non-ABS-assisted hard braking marks. I suspect you'll see there aren't really any differences between the two.