I have tubular rim that is clearly out of round. I was involved in a crash where the wheel slipped out due to gravel in the road. There was no fast collision into any object. I rolled the bike and crashed it down sideways. As a result spokes lost tension completely. I was not going fast nor did I hit a deep pot hole.
Is this a hopeful scenario where I will only be needing spokes and a truing?
Completely depends on the severity of the buckle. If it's more than 5-6mm (possibly 7-8mm if you're very lucky) out of round then I would be inclined to say your wheel is toast. Without holding the wheel or at least seeing some good pics it is impossible to accurately comment.