It = single and legends= pluriel. It should be these or those legends.
I think so too
"These are all legends".
It's (single) all legends (plural)
These are all legends, would be כל אלו אגדות. Like in an array of books, choosing a small section of legends.
אם תרצו אין זו אגדה
Ze hakol agadot.
The it in English is a dummy pronoun for the presentative. Like in "it is raining" there is no referent for "it". Similarly, zeh is singular looking but referencing a general thing.
Or: This (זה) is not a real history, it's all a bunch of legends. It's OK in English too, to mix a singular subject with plural complement.