Proper deck post installation
If I understand your description you're concerned because the posts are encased in concrete with no gravel under the posts.
It would have been good to have gravel under the posts for drainage but it's not critical. Of greater concern to me is the several feet of soil they placed around the posts above the piers. Encasing the posts in concrete isn't necessarily a problem but in my opinion you want to avoid having soil directly in contact with the posts.  Microbes in the soil and moisture in contact with the posts will accelerate rotting so you want to avoid that. There are various ways to anchor deck posts and provide for drainage away from the posts. I have seen deck posts done just the way yours are - it is structurally sound but I don't like it because of rot issues. Given that your piers and posts are completed I'm not sure what recourse you have. So let me show in the picture below the way I prefer to do posts and one possible remedy for your situation.
The illustration on the left is the way I prefer to do deck posts because it keeps the post above grade while still providing support. The one on the right might be one remedy for your situation. It would mean removing the soil around the posts down to the pier and then backfilling that space with stone for drainage. It's not the ideal solution but it will give the posts a longer life.