That is an older system that has been employed for many years and is a recommended system by NOFMA. In my area, I cannot find the asphalt mastic but I understand it is available still in other areas. The material costs are way less than the newer moisture barrier systems and adhesives. I would not rely 100% on an asphalt mastic adhesive however and would nail as well, using those fluted masonry nails. I have used those many times in the past and they work great, IMO. I drive them with a 2 lb. sledge. But you can use your powder activated nailer and nails as well. But your correct in your assessment. One "glues" down the 15 lb. asphalt felt paper with the asphalt mastic to the concrete using a small notched trowel to spread the mastic. Then you do it again, gluing a second layer of felt to the first, using the mastic. Then you use a bigger notched trowel ( 1/4"x1/4"x1/4" ) to glue the plywood to the layers of asphalt felt. The problem with this system is it is slow because one needs to wait for the mastic to "flash" and that takes awhile. From 1 to 4 hours. And you have lay two layers of felt, three layers of mastic, etc. For us pros, time is money. I'd rather go with a "system" from a manufacturer that has a warranty and is simpler and faster. Plus all that black mastic? It's messy!