You're right that fossil fuels are contingent. They exist because of a big evolutionary coincidence that might not be repeated on other worlds.

It's possible, though, that aliens might not need fossil fuels to get to space. If they had more efficient photosynthesis, for example (ours is <5% efficient), living plants could have the energy content of gasoline. They might eat gasoline. All their fuel usage would be carbon neutral (CO2 pulled out of the atmosphere). They would be way ahead of us.

Or it's possible that we're the most advanced because fossil fuels are necessary and nobody else got them.

Wait, can you elaborate on that? Cursory glance at google suggests there can be sufficient other sources of hydrocarbons aside from dinosaurs being wiped out by a meteor.

Plants evolved lignin well before fungus figured out how to break it down. That gap in time is when most of our coal was formed.

How efficient would these plants need to be?

Afaik the iron age depended on charcoal, not on fossil fuels. Europe has seen a wide deforestation due to it. Industrialization might depend on fossil fuels but not an iron age per se.

The idea of the 'great filter' isn't that a necessary item's absence prevents advancement -- the idea is that through technological advancement, something happens to destroy most civilizations along the way.

but stated it as you have, there's a huge number of filters and requirements, and they are all over the place, and that's definitely true.