Imagine
Instead of going to the Jarl to ask to send guards to riverwood, you join the stormcloaks and take over Whiterun.
There will be a new Jarl, does that mean that you will have to ask the new Jarl to send guards? And that he will ask you to get to that tomb?

You have to do dragon rising before doing the civil war questline further.

Oh
That explains it.

I've only sided with the stormcloaks once on about 4-5 completions of the civil war across my characters... I'm curious. Does Riverwood then get occupied by stormcloaks instead of Whiterun guards?