Tunneling in war was first done centuries before WW1. First known incident is from Roman times.

And even doing it with explosives in trench warfare wasn't new.

https://en.wikipedia.org/wiki/Battle_of_the_Crater

Which is likely why the article says "an attempt to master the art of mining and reinvent it for the age of the machine gun." Italics mine.

The need to master mining in the first place is because "in the belief that siege warfare tactics like this were a thing of the past, the Royal Engineers had wound down their military mining section."

Wasn't there a bit in the Iliad about sneaking into Troy through a tunnel or ditch?

(Here we go, Odysseus and Diomedes steal the Palladium, use a 'secret passage'; apparently not in the official Homer version of the story https://en.wikipedia.org/wiki/Palladium_(classical_antiquity... )

Would tunnelling robots make military sense in the 21st century?

No. We have airplanes now, with precision guided bombs including bunker busters.

I'm imagining a future war where there will be robots bombing robots trying to dig underneath other robots while AIs try cracking each other as well as the robots.

All the while, the Humans on either end of the battlefield will be participating in the war as if playing a game. I'm not sure if I want to picture the poor suckers mixed in the middle.

You can deny aircraft from an area. A TBM dragging a thermonuclear weapon might be a bit tougher to guard against.

With modern seismic detection devices, you'd be able to detect that from great distances. Not to mention those dig very slowly.

on the small end of asymmetrical warfare, they always make sense

I just imagine that this was the dream of hundreds or maybe thousands of years of sappers, carried out by 18 men.

This clearly inspired the early part of "The Proof House" by K J Parker (part 3 of The Fencer Trilogy)

The BBC show "Peaky Blinders" includes flashbacks to fighting in the tunnels at the Somme.