Sealing siding at foundation wall
I have a similar situation, cedar siding that comes down to within a foot or two of the ground, depending on where around the house you are.

I kill the ants, foundation spray, shake pellets. Ants are, for the most part, smarter than I am and will always find a crack or crevice I didn't and they aren't afraid of heights.

I would never seal my siding at the bottom. Depends on different situations, the house is 60ish years old. Well before things were tight and meant to be tight. I don't want to trap moisture in a space that was not built for it and where this house needs to breathe or weep excess moisture. Could be wrong, but there is a reason you don't seal stucco there either, water needs an out (I am North East if location and weather matters to you).