I think they can imagine non-imperial societies; the argument is more that non-imperial societies will be destroyed by imperial ones, so you either compete as an imperial state or become part of someone else's empire. Same applies to the dark forest theory.

This is different from capitalist realism, which:

• Views AES states as simply another kind of capitalism ("state capitalism")
• Views pre-capitalist economies as, you guessed it, pretty much capitalism (often the markets = capitalism oversimplification, as well as ignoring the communal aspects of societies)
• Predominantly assumes capitalism as an economic system in fiction