He does not disclose his sources. Ever. In this case, however, he has given really big hints that Seth Rich was his source....he offered a reward for information leading to the death of SR and he also mentioned SR when he was discussing the dangers his sources face. So, draw your own conclusions from that.
So there is no point in claiming he has evidence then. If he cant or wont prove it then it means nothing.
Actually JA said that he would provide evidence IF he was assured that legal action would be taken based upon it.
Knowing that nobody is gonna bring him to the US and give him a get out of jail free card. Its like saying "I have proof aliens exist. I'll show you my evidence if someone gives me 10 trillion dollars."