I have a small closet that's roughly 4x4 (maybe a little less), I have engineered hardwood, approx. 5in W, 1/2H and since the area is so small and nobody walks on it (or really even uses the closet), can I use this wood glue from Home Depot called Roberts 1407 to glue directly on the concrete.

The concrete surface is clean and flat.

Thanks

The glue will work on concrete. Use the quarter inch trowel.

Thats the weakest adhesive known to man, garbage. I'd only use it over a wood substrate. But since your not going to be doing a huge area it won't cost much to replace it.