The upstream capacity has little to do with that specific gadget.
And as always, assuming they are honest is pretty silly. With this sort of gear, do your due diligence and try to verify all claims.
NEMA 5-15 standard is intended for 110-125V only. It is also rated for 15A only.
It is very common for multiple 5-15 outlets to feed off one 20A circuits.
However per code, if you need to feed a 20A device, there must be only one outlet per 20A circuit, so a duplex receptacle would require two completely separate 20A circuits.
For 220V use, code requires a NEMA 6-15 if amps drawn is 15A or less, and 6-20 "T-slot" outlet for 20A
Such cheating might fly in third world nations, but in developed countries it sure smells like scammer city to me, and it is very likely dangerous to take their word on anything.
In more civilised well regulated countries than the US, you could be arrested maybe even jailed for carrying such a device.