Yes agreed, 30 minutes would seem to go against the Overground 'turn up and go' mantra.
Another thing I have just picked up from the TfL document re night services - we know they are only going South as far as New Cross Gate but the reasoning is this:
Other LO services operate mainly on Network Rail infrastructure
where maintenance is usually carried out on weekend nights. Routes are shared
with freight trains which operate overnight during the week, limiting access to the
network.
Maintenance we know about but we already lose the Overground day services for whole weekends for this so it seems strange to not provision night services on that basis.
Am also curious to know which freight trains these are which share ELL routes. Am generally a sound sleeper but am surprised that I have never noticed these.