Should I install a floating floor or a staple down for engineered hardwood?
I have placed Dricore or its equivalent on 2 different jobs. Both are holding up fine, neither felt hollow or creaky. One was in a basement slab, the other was placed on a slab on grade such as yours. The only difference is one had 4" to 10" wide plank flooring from recovered barn wood, the other 2 1/4" strip.

While the subfloor was being laid, I glued the edges of all the pieces before placing. even though it is not required. Also the perimeter had a gap to the wall of at least 1/2". It is laid as a floating floor. I really don't see any problem stapling down your engineered floor to that. When the strip floor went down, the staples where checked for length so they did not go all the way through and pierce the plastic matting.