Yeah, my thoughts are this guy doesn’t know what he is talking about. First, sand doesn’t need to be changed, although it might need a deep cleaning. The cartridges needed to be hosed off. And that blue mineral stuff is copper-based which will lead to staining and should not be added to a pool.
Please don’t be tempted by this kind of thing. You are almost there!
Sand is not particularly expensive, but it does have to be a certain size to work in a sand filter. You can get #20 silica sand for around $25-30 for a 50lb bag. A 24" diameter sand filter takes about 300lbs of sand. Changing out sand is a bit of a pain as you need to scoop it out by hand being careful not to break the laterals, or use a shop vac. If you break a lateral then it must be replaced for the filter to work properly. Then you have to find someplace to put 150-300lbs of wet sand.