I think OP may be implying the fossil fuels are necessary for the "industrial revolution" part of a civilization's development -- and that industrial revolution is a gating factor for achieving spacefaring technology.
Yes, it's this. After the collapse of the western Roman empire, the standard of living dropped to bronze age levels in many areas, well below where it was before the Romans took over. Assuming a similar decline after modern civilization collapses (to say pre-industrial revolution levels), I don't see how it's possible to get back without the easy surplus energy. It's much more complicated to build batteries or nuclear or solar than it is to make a steam engine.