What does 'imaginary' mean in the passage?

Question

If these ideas is assumed to be about something like soccer rules, why are these rules imaginary, not concrete?

EVOLUTION DID NOT ENDOW HUMANS with the ability to play football. True, it produced legs for kicking, elbows for fouling and mouths for cursing, but all that this enables us to do is perhaps practise penalty kicks by ourselves. To get into a game with the strangers we find in the schoolyard on any given afternoon, we not only have to work in concert with ten teammates we may never have met before, we also need to know that the eleven players on the opposing team are playing by the same rules. Other animals that engage strangers in ritualised aggression do so largely by instinct – puppies throughout the world have the rules for rough-and-tumble play hard-wired into their genes. But human teenagers have no genes for football. They can nevertheless play the game with complete strangers because they have all learned an identical set of ideas about football. These ideas are entirely imaginary, but if everyone shares them, we can all play the game.

"Sapiens" by Yuval Noah Harari

Answer 1

The rules of football are not born into humans the way play-fighting is born into puppies, nor are they an inherent quality of the universe. The rules are only as solid and as meaningful as our mutual agreement that those are indeed the rules to some activity called "football"; they exist only as a shared mental construct in our imaginations. The rules are imaginary because they don't reflect a physical or in-born trait of humans.

Imagine if an American who has only ever played the rugby-like game they call "football" walked into Salamanca and tried to play "futbol". He has a completely wrong idea about what "football" is because his mental construct is the wrong one.

Yes, we can write down and share the rules, but they're still basically arbitrary. We could change the rules of football completely and it would only require getting everyone to agree to it. In that sense, the rules are imaginary; they are created out of our collective imagination.