It might just about meet regs on technicalities/or differing interpretations, but just because the regs don't specifically ban something doesn't mean we can't decide something is potentially dangerous.
I'd have no qualms deciding that I didn't like the environmental influences - steam and moisture. Also, as already said, the cupboard is easily left open, or easily opened while using shower, and shower spray becomes a huge concern.
The OSG mutters about an airing cupboard in a bathroom not being in zones. But that is a complete wall and door.
A cupboard isn't listed as something that can be taken into account to limit a zone, and it says a door has to "effectively" limit the extents of the zones and I'd argue this one doesn't.
The zone clearly extends to under the cupboard and there is nothing that effectively limits the zone, so for me it's in zone 1.