I don't have any experience with that brand. Although, I've been using a similar soil (Great Lakes) for the past two or three years and love it! I've never gotten away without having to supply some sort of supplements, so I guess you could say that claim of water only is Bu!! Sh!t. What I can tell you is that I use far less nutrients and have had great success with my grows!
What I find works best for me (when using water only soil) is somewhere between weeks 2-3 of Veg, I'll give them a half dose of Fish Sh!t Organic Soil Conditioner in pH balanced water. And I'll continue that, every other watering throughout Veg and into preflower. Once in full bloom, I switch over to a simple Compost Tea every other watering and that's it.
So in closing, water only? I'm not sure about that. Although I do know, that I'm spending far less time mixing, flushing and buying all kinds of nutrients, with great results!