Not necessarily. The notice is probably to help them protect their aquarium grade products, which are the same only much more expensive and with a different bag.
It may be a disclaimer of sorts to ward off any lawsuits that could arise from it's use in aquariums with not so good outcomes. Companies today are acutely aware of potential liability.
Silica sand works fine with aquaria by the way. Just doesn't offer any buffering capacity. But who cares if you run a Calc reactor or watch the levels yourself?
squeezstix, the play sand Southdown makes is still the calcium carbonate kind. It just isn't bagged for aquaria and has a disclaimer to make the consumer think that he has to pay 40 times as much.
But I have used silica based and it worked fine. No diatom blooms, contrary to what I expected.