Exactly. The visibility or otherwise of abdominal muscles are not an indication of strength. It's possible to have a six-pack while lacking both strength and fitness; bodybuilders etc call it being, "skinny-fat".
In answer to the original question: yes, slendertone is a load of bollocks.
A sorry excuse for hard work IMO. Nothing substitues for crunches and preachers in the gym. Id feel 200 crunches on my abs more than i would 10 minutes on a slendertone on the maximum setting, and i have done it.
And like has been said, 90% of your battle is your diet and you Body Fat %.
I bought one once as an experiment, at a time when I was laid up with an injury. I knew it wouldn't create a six-pack, but I was curious to see if it made any detectable improvement in muscle tone. It certainly causes the muscles to work, and the post "exercise" after-effect seemed to be the sort of mild stiffness you get after a few crunches. I probably exceeded the manual recommendations of time vs setting by a factor of ten, and stuck with it for a few weeks - but the end result probably equated to less than 5 sit-ups per day... Definitely a gimmick directed at the sort of people who think they can look like the model in the brochure without doing enough exercise to break into a sweat. There might be some sort of physio use for this muscle stimulation equipment, but its worthless as a crunch substitute.
Theoretically some types of these work(the pad types with electro-conduction-medium) but only under some very certain and specific conditions. The chief of which is if the impedance of the tissue surrounding the targeted muscle isn't too high; they were first designed as therapeutic devices for athletes in recovery where limited movement was a factor.
The other issue is the mechanics of the human body. Muscles twitching due to electrical inputs do nothing but cause a localised spazm if there is no mechanical resistance offered. This done incorrectly can cause muscle damage.
In the case of belt worn versions; if you already had decent muscle tone and body fat low enough to start showing striations across your abdomen, you may get the reputed benefits if you were able to keep the muscles group under tension for period of the exercise.