I am not an expert - but have been on construction sites for work.
On concrete slab the big construction companies will identify the high spots with laser, then level it off by grinding away the top layer with abrasive.
Dirty, noisy, and disruptive, and slower than an impact hammer or windy chisel.
20mm is a lot of material.
My guess is that he will dig it out manually to 30mm depth, then re-lay the top 10mm