What do they do that is more impressive than routine tunnels built in cities for the last few decades (or even between different soverign countries under the sea!)?

Their main differentiator is cost. The Boring Company bid $48.7 million for the initial LVCC loop. The total cost to complete it was $53 million. The second cheapest bid was Doppelmeyer Cableliner, which would have built a people mover for $215M. The people mover would have had about 50% more capacity per station, but at 4 times the cost.

Tunnel cost is mostly dependent on the volume of material removed, which means that cost goes up linearly with length but with the square of the tunnel diameter. Trains and people movers tend to require significantly larger diameter tunnels, so their costs tend to be much higher. Also Boring Company tunnels don't need much infrastructure in them, so they save money on rails, high voltage power systems, rolling stock, etc.