API and the information it can pull are totally IC, in my opinion. Especially mails (unless they're obviously OOC either by marking or by content) assets and kill reports. Chat logs can be forged, yes, but it's not easy to do properly, especially in bulk, and logs aren't hard to verify either.
As we can't generate (read: forge) kill reports ingame (yet), I'd argue that simply producing one of those ingame is the simplest way to call BS on someone claiming a kill/loss never happened.
In general, though, it goes back to a lesson all of us should know already: don't say or do things around people you wouldn't trust with that information. You don't have any control of it once you hit the Enter key to send the message, so once you do, be prepared to suck it up and deal with the consequences.