Because Afrikaans commonly forms compound nouns as a single word.
Together, skatkis means treasure chest.
This is very normal in Afrikaans. English often uses two words where Afrikaans uses one compound word.
It is grammatical and clear.
However, if you mean that the treasure chest has a false bottom built into it, Afrikaans speakers might often say:
Die skatkis het 'n vals bodem.
That means The treasure chest has a false bottom.
So the given sentence focuses on the location of the false bottom, while Die skatkis het 'n vals bodem focuses on the chest having that feature.