Consider the following:
Tzeentch is arguably constitutionally incapable of not backstabbing everyone including itself and its servants
Via Ahriman, Tzeentch is directly at war with Cegorach and the clowns.
As 'fellow' Forces of Chaos, Tzeentch and Slaanesh are co-belligerents in the clowns' war against Chaos.
After Cegorach steals a Solitaire's soul from Slaanesh, it has to sneak away/escape somehow
Now suppose, hypothetically, Cegorach were to decide to use the Realm of Tzeentch as part of its escape route. Tzeentch would ultimately have two choices:
Kill/capture/corrupt Cegorach. This is front-stabbing one's enemy.
Let Cegorach get away. This is backstabbing both Slaanesh (the co-belligerent who owns the stolen soul) and Ahriman (a favorite champion whose goal (which possibly requires entrance to Black Library) Tzeentch permanently holds just out of reach).
All I'm saying is, Cegorach has probably used this route more than once, with Tzeentch only ever offering token resistance.