Can I play Snowborne Scout as my first Rohan ally in the round by resource matching with Elrond when I have Spirit Theoden?

No, because since the cost is reduced to 0 you are not actually spending ressources from Elrond's pool and thus his ability does not apply.

(I happen to think this ruling is stupid, but I believe that is the ruling)

Yes, there is an odd feature of the rules for playing cards in this game where there is a distinction between cards that cost resources (you must spend resources from the correct resource pool, but there's no explicit requirement to control a hero of the same sphere) and zero-cost cards (there is an explicit requirement to control a hero of the same sphere).

Originally it didn't make a difference because there used to be an implicit requirement to have a hero of the right sphere to play cards that cost resources, but Elrond and Good Harvest removes this implicit requirement.

It's one of the rulings where, if you read the rules carefully, the ruling is the only logical one that can be made, but you're left feeling they'd have been better off just changing the rules rather than following them.