You are right that tremolite is in the amphibole group of asbestiform minerals, which is the nastier one, having as it dies smaller needle like fibres as you say.
However, if we assimilate it to crocidolite or amosite, both amphiboles that we know a lot about, they were used mainly as a binding product to give strength to something else, for example cement or rubber, so were added deliberately. If I snap a piece of AIB (asbestos insulating board) as I sometimes do (wearing the right PPE, then I see along the broken edge bunches of fibres very visible, put there on purpose.
In this case the sand has been extracted from a pit where there is also some tremolite, so the odd fibre ends up in the sand. by no means can all fibres be inhaled, they have to be tiny, those are generally produced when you snap bigger fibres and tiny bits break off, say in cutting, breaking, drilling a product containing asbestos, that's not going to be the case with this sand.
I really doubt even if the sand had been thrown around that the 0.1 f/ml would have even been approached, let alone crossed.
To put it in context, if say, we removed a ceiling containing amphibole in a school, so long as the test showed less than 0.1 f/ml when all cleanup was done, the school can open, that doesn't mean there is nothing in the air, just that it is within the level considered acceptable.
Sadly in the past asbestos was used so widely, with no precautions, even though the dangers were first identified by the ancient romans, that many deaths occurred - most, if not all from chronic exposure, I suspect that the teacher pinning pictures to a wall got their dose unwittingly elsewhere as hundreds of factories used it venting the air out over communities.
We are in a much safer place now - try not to worry about your DC, their likelihood of ill health due to this is vanishingly small, likely zero. There are many worse hazards out there.