Volcanic rock dust is a means to "mineralize" soil. Great for a long term soil building strategy. OK for containers. Mostly because microbes will colonize the particles, and if they can break them down, will release what they're containing. However, while I advocate the use of rock dusts in soil and container mix building, I certainly wouldn't put any stock in the marketing claims. People will tell you a lot of bullshit to sell you dust and dead bones.
One of the very reasons that I hate bonemeal so much, is that it's a byproduct of the meat and rendering industries. There's not real use for bones, in alot of places. So they repackage it as something else, and sell it for a premium. In truth, it's not the best source of phosphorus, and it's certainly not readily available. (and will most likely never become so, in a container)
Here's a cheap trick, if you want to save money on volcanic dust, and get some effect. Go out into an empty field. One where not much of anything grows. Look for a patch where weeds are growing. Find the healthiest and most lush weeds. Whatever they're growing in, take a couple buckets full of it, and mix it with your soil. Whatever is giving them an advantage over the rest of the barren landscape, you can be sure that if it's a real advantage, you'll get it, too.
That being said, don't pay too much for mineral dusts. If the price is right, OK...