Silica dust in kitchen countertops
Pretty common practice for most trades not to clean after themselves.  Even the trades that do clean after themselves I have a hard time imagining cleaning the underside of the slab.
My experience is the same as yours.  If you touch the underside of the slab you are likely  to come away with dust - unless you personally cleaned it.
I wouldn't worry about it.
Silica or silicon dioxide is regulated by the FDA to be no more than 2% by weight of any food sold.  It is unlikely whatever did drop into your plates was more than 2% by weight of your food.
The danger with silica is inhalation but the concern is really around industrial exposure where you are working with concrete cutting/grinding or mixing and there are visible clouds of it in the air and you are breathing these clouds on a semi-regular basis.
I find it hard to imagine that you could ever expose yourself to enough of the dust left under the counter for it to have an affect on you.  You probably get more exposure from buildings being demolished, city workers doing work on sidewalks/roadways, renovators mixing mortar or concrete or thinset in your neighborhood, etc.
FDA regulation