If you presume that Vaneigem is fundamentally a mystic (a person whose experiences are so sublime as to be nearly impossible to communicate to another person who hasn't had similar experiences), then his abstract excursions into poetic prose can be approached as symbols and metaphors rather than proper descriptions of a more or less agreed-upon reality.
Like most of the other Sits, he reveled in Hegelian dialectics to the point of near-Proudhonian contradiction as a goal. "Mobile within immobility" is a perfect example. It's not meant to describe a path, but is meant to fuck with your perceptions of reality, much like a Zen koan.
However, I can also understand the entire phrase as you quoted it to indicate that in unitary societies (ie, those where there is no dialectic, no movement of the spirit [Hegel], no development of the productive forces [Marx], and no progress toward liberation -- that is, they are static, immobile in revolutionary terms -- like feudalism), their inhabitants lack a vision of a liberated future (The Revolution), and so are immobile. The only motion is cyclical, following the seasons. For anyone interested in The Dialectic, this is intolerable precisely because there is no goal, no aim, no vision. Such societies produce only enough to replicate themselves, no more. For dialecticians, this mean there's no real culture, no real art (to be realized and suppressed). There are no stages of history because there's no history. Such societies are inward-looking, and therefore parochial and xenophobic. The visceral hatred of Marxists for peasants comes from this, but it's also older than that.
Hope I haven't just confused things more...