BTW double conversion UPS often have less protection than power strips. Anyone can read its specification numbers. Most do not. How many joules does that double conversion UPS claim to absorb? Notice a tiny number.
you assert that any surge seen on input of such UPS coming through AC->DC->AC transformation survives and comes out in original form??
No, but it's possible with a big enough surge to overload the circuitry and pass part of the surge along, or for the voltage to just jump the isolation.
20k volts isn't exactly HIGH for a lighting strike.
BTW, the APC unit linked in the message above this isn't double conversion, but while a double conversion UPS (don't think APC makes ANY of those) can probably soak a bigger surge without passing it, even double conversion has LIMITS to how big a surge it can soak.