I wouldn’t say Trump was joking. He was scoffing. He was mocking the leftist conspiracy theorists.
Here is a clip with context: https://www.youtube.com/watch?v=WbfUjotYmUo
The 30,000 missing emails he mentions are the ones Clinton deleted before she handed her personal email server over to the FBI. The server was no longer online at the time of Trump’s statement.
If we were to assume, as you do, that Trump was not simply mocking the conspiracy theorists but, instead, was serious, he would have been calling on Russia to release any of the emails they already had.
What’s your problem with that?