Why do you think it was in the file for D's to release? Kash didn't say it was. He implied it wasn't.
The fact that Biden didn't release it does not mean it's not real. That's an inconclusive bit of information. If the claim were that it's in the file, I would certainly wonder why Biden didn't release it, but I also know that there are potentially other reasons for Biden not to release, and those reasons could outweigh the dirt on Trump.
"If there was dirt in there, Biden would have released it" is a conclusion made by people who haven't thought it through.