Found more info:
Heating Help
"Domestic water is heated instantaneously as it passes through the external
stainless steel heat exchanger. A flow switch turns on the small circulator
supplying this heat exchanger whenever there’s a demand for domestic hot water.
Hot water from the top of the tank flows through the primary side of the heat
exchanger and instantly transfers heat to the other side. This heat exchanger
is easily replaceable (and recyclable) if ever necessary due to scale or other
issues. The minimal quantity of domestic water residing in this heat exchanger
(probably less than one pint) reduces the possibility of legionella
growth."
A comment from Fortunat:
Regarding heating DHW in a single pass flat plate HEX, I
can't seem to find it right now, but I read a good paper (maybe by ZfS
- Rationelle Energietechnik GmbH in germany) about the control
challenges of this arrangement in large SHW systems. I suppose if you
just bang the pump to full speed anytime the flowswitch on the potable
side registers any flow it would work ok, but you'd be mixing the tank
up unnecessarily. The germans tried variable speed pump based on
heat exchanger exit temperature and (not surprisingly) found the
control to be quite challenging because of the odd and spikey nature of
DHW demands. Have you actually tried it?