It is indeed "the old lie" according to Owen. Full quote:

If you could hear, at every jolt, the blood

Come gargling from the froth-corrupted lungs,

Obscene as cancer, bitter as the cud

Of vile, incurable sores on innocent tongues,

My friend, you would not tell with such high zest

To children ardent for some desperate glory,

The old Lie; Dulce et Decorum est Pro patria mori.