> allocate the whole space

what do you imagine allocation is in this context? it's just a set of written blocks that mark address ranges and other metadata for the OS filesystem driver (all encrypted).

firmware metadata will leak the fact that there is churn in the address range where a potential hidden volume lies. the churn will be inconsistent with filesystem activity that would be present in its absence.

it's not just SSDs you need to worry about either, HDD firmware also keeps metadata, some of it could be be proxy to churn by region.