can it not also be "it's" as opposed to "this is"?
Yes it can. Flag it with the "Report a problem" next time you see it.
Could it also mean that it is a diary?
As it's mentioned on this page, yes it can also mean "diary".
It's my diary is not accepted.
A diary may or may not be secret. This one is. That's why your answer wasn't accepted.
"It's my diary" will be just "c'est mon journal" in French.