Zombie gamemode abilities

[InjectuousPotato (Incandescent)]

@wiki

Time Warp

S S

Takes you back to where you were previously, including your hp.

W S

Forces enemies to stay still for a period of time. Debuffs allies with slow but buffs yourself with Swiftness and Panic.

Upgrade Effect

• Increases the length of the time stop
• Increases the length of the debuffs on other players
• Increases the length of the buffs on yourself

According to this, there is no S X for the time warp ability.
Since S and X are close to each other, it was probably just a typo wherever you got that info from.