I guess this is the kind of thing I've heard about - no experience myself, either with ABS or with proper dirt riding.
Richard, has the effect you described happened to you?
No. I don't ride in the dirt, but I have talked with people who do and read lots of magazine articles written by people who do a lot of off-road riding. To a man, they all say that ABS can cause the bike not to stop on loose surfaces, especially when riding down a hill. The ABS system feels the wheels rotating at different speeds and "thinks" they are slipping so it cuts braking until the wheels start turning at the same rate, which might not happen when one wheel hooks up and the other wheel doesn't on a loose surface with varying traction.
That is why many ADV models now coming on the market have a switch that allows the rider to turn the system off. Others have a setting for off-road riding that turns off the system for the rear wheel, but keeps it working on the front wheel. You will see these features showing up on most current European ADV models such as BMW, KTM, Ducati and Triumph brands. They wouldn't be offering switchable ABS systems unless they felt their customers needed or demanded them.
In my case, I have owned BMW motorcycles that have had ABS for the past 12 years and not once has the system activated and done anything other than increase the complexity and expense of my braking system, along with resulting in fluid replacement costs that had to be performed by a BMW dealer at a 2-hour labor charge. So for me, ABS seems to be a waste of money. Having been riding for 57 years without it and I have only experienced a rear wheel skid once, so it just isn't something that makes sense to me.