Concrete countertops ghosting fix?

I have a feeling that your ghosting is here to stay... I think the skim coat idea is not good, unlikely it's going to be very durable. I'd consider staining it to a more uniform color and then sealing it.

Would a skim coat (specifically, this product) cover it up or would the ghosting still show through?

Ghosting is caused by matrix changes. A skim coat will have a different matrix and should cover it perfectly well. That particular product has good reviews (on Amazon), so I would bet on it.

Ghosting is common with microtoppings, but having a ghost (a slightly different underlying matrix) is not enough to cause this minor problem to get worse. Having major differences like cracks, seams, or significant porosity differences on the table/substrate would create ghosting in a skim coat.

Letting the table dry sounds like sage advice, because the product was made to go on dry surfaces.

What I would do is put some 2x4" Southern Yellow Pine or even better Oak framing under to brace those flimsy compressed wood cabinets. Otherwise they will bow out over time.

Then I would get an epoxy based coating for concrete. They have these in many types, with or without stone, in all sorts of colorings and will look really good and hold up for years. When done you will have a counter you will be proud of as these epoxy coating look really good.

Usually these epoxy products are two or three parts. Epoxy, hardener, and sometimes a ground stone you apply over the epoxy. The epoxy will vary in price from around $40.00 up to maybe $120.00 a gallon depending on what look you go for but you will probably only need a gallon to cover that concrete counter.

Lifetime of these products vary, but I would expect you to want to remodel the room long before the epoxy needs replacement.

In the future when you make a concrete counter make sure to use a sturdy cabinet(s) as concrete is heavy and modern compressed wood cabinets can't take the weight an old wood or even plywood cabinet could. You should also make sure to tie down the reinforcing mesh so it stays centered in the concrete.