Why do all gases occupy 22.4 L [per mol] at STP?
The question is based on a false premise. Only ideal gases are guaranteed to occupy 22.4 L/mol at STP. There are many gases that are not ideal.
So going by this, all gases should occupy same $x$ L at some other temperature-pressure conditions. Is this true?
No, again this would be true only for ideal gases, not all gases.
At STP, many commonly encountered gases on Earth behave nearly ideally, but certainly not all. For example, vapor of acetic acid and other simple carboxylic acids show strong deviations from ideality.
Wikipedia is a good place to read about the assumptions of the ideal gas law and the kinetic theory of gases. The reason that ideal gases share a molar volume of 22.4 L at STP is not related to diffusion, but rather to the assumption of: (a) non-interacting gas particles that are (b) effectively zero-sized relative to the mean distance between them.
You can read about equations of state for non-ideal gases on Wikipedia as well. Popular equations to describe non-ideal gases include the van der Waals equation, the Redlich-Kwong equation, and a whole slew of others listed at Wikipedia's real gas page.