RWBY - In Ozma's defense

It's also worth noting he asked a literal genie for answers.
If you don't word it right, you won't get the info you want.
If I remember right, he asked "how can I kill Salem."

Operative word there is "I."
He, by himself, can't do it.
That doesn't rule someone else or a group.

Of course, the implication is that she's truly immortal, but that's an implication and not necessarily absolute based on current info.

Ruby : "What Ozpin is hiding from us ?"
Jinn proceed to show to an horrified team RWBY and Co Ozpin's fetish and internet history.