I would not recommend this, with concrete timing is everything. These forms are made of plastic and trying to force them into the cement would probably break them. The concrete would have to be very wet, which would hurt the overall strength of the slab. Rubber stamps are available just for doing this very thing. Pour slab; let it tighten up a bit, put on a release agent so that the concrete does not stick, lay mat down and force the pattern into the surface with a tamp. Again timing is everything; if you get behind you will be beating the stamp into the surface. Been there, done that!
With that being said, it would be possible to take one of these forms, build a wood form that matches the outside shape, lay the plastic form over the wood form, and proceed as directions indicate. This would allow you to make a stepping stone that would be approximately 2’x2’x4”. Movable but heavy. Only drawback would be that the plastic form at 2” thick would be too deep into the concrete, making the stone weak. Therefore, you would want to cut the thickness of the plastic form down to no more than ¾” thick. Now you would be able to make stepping-stones that could be moved into place that would be strong. A lot of work, but as anything, do it right the first time and it will last a long time.
BTW, if using bag/sack type premixed concrete; add approximately a three-pound butter tub of pure cement to 80# bag. This will increase the strength of the mix and make it last longer.