As I said, 0.4 bar (~6 psi) above atmospheric pressure is a fair bit. If I recall, pressure cookers generally run at a bit under 1 bar above atmos, and they most certainly "WHOOSH" (this time in uppercase
) if one opens the valve 
How could such a thing be possible?
Inferred, maybe incorrectly from @JohnW2 's comment saying the low powered ones store it hot. Does that mean the high powered ones dont?
Yes- but, as I implied, they have to be very high powered. You are essentially talking about the equivalent of an electric shower (which can produce pretty hot water 'instantly') - so probably in the ballpark of 10,000 W (10 kW). For a given desired hot water temperature, the lower the (electrical) power, the lower will be the flow rate.
As above, one could certainly get very close to 'boiling' (actually 'boiling' might produce difficulties, due to the steam) - but, as above, one would need a heater at least in the ballpark of a shower (i.e. 10+ kW) to get a sensible flow rate.
I think you're probably taking 'instantly' a little too literally - even a shower does not produce 'full temp' water 'instantly' when 'opening the tap'- at the very least there will be a little cold water to be initially 'flushed out', and the 'works' to be heated up.
Totally off-topic, but that reminds me of something I experienced a few decades ago.
It's not - but, as I said, there's no reason why the same technology could not be used to get water as close as one wanted to 100°C. Showers have built in mechanisms to prevent one turning the temp up that far.
It varies quite a lot from tea to tea, but I think you'll find that the 'recommended' optimum temp for brewing tea varies in the range 80-96 °C (I think around 90° for many teas), and the 'experts' certainly all seem to say that one should never use literally boiling water - and such temps are quite attainable with shower-like technology (indeed, quite probably with a shower, if one could disable its temperature-limiting measures).
I think that's what most of us have been brought up to believe - to the extent of the 'warm the pot' advice.
Many moons ago I attended a fancy "tea tasting" event (a sort of glorified up-market 'afternoon tea'). We were presented with a series of ('leaf' of course!) teas. Each one came with a jug of recently-boiled water with a thermometer, and we were told (different for each tea) what temp it had to fall to before we poured in on the leaves - and also stopwatches, to tell us how long thereafter we should 'pour it'
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