It's not a cheat. It's explained in the game's manual (the Official Guidebook [1], just search for Engrave).

It's been nerfed since 3.6.X as well. Now it can no longer be used for fighting, only escape, and attempting to fight while standing on it will make you "feel like a hypocrite" and deduct 5 from your alignment score.

[1] https://www.nethack.org/v500/Guidebook.html#toc_4

I know it's not, but I don't understand why it has to exist.

There are a million other wacky things in the game. Nothing about Elbereth seems out of place or unbalanced (since it’s been nerfed anyway).

They added the ability to apply your money to flip a coin in this version! Why does that need to exist? Because they thought of it!