Normally when this "added concrete" happens (due to his mistake not taking note of the measurement), when they "add the concrete", they will for:

Option A: redo the whole portion.
Option B: to add, then they need to hack surface of the perimeter (inner side) of the opening, make surface rough, then insert rebar with Hilti (or equavalent) to hold tightly the bonding newly pour CONCRETE, NOT CEMENT PASTE.... let it dry for few days, to get maximum bonding. Then apply tiles on top to get more bonding.
Option C: apply option B, with metal supports at bottom.

If your case, doing heavy stuff cooking....better have stronger supports!

But 1stly, have you told the chinese guy that he must construct the conc top (with stronger rebar, stronger grade of conc mix...etc) according to your spec??

Just sharing...thanks