The United States does allow people to run hubs on other Z wave frequencies, unlike some countries which restrict use to the zwave alliance-assigned frequency for their region.
So it is legal and possible to run a 868.4 frequency hub with 868.4 frequency Z wave devices in the US. It’s just not what most people do.
(For one thing, all the zwave light switches made to run on typical US wiring use the US region frequency.)
So the company isn’t technically incorrect that their device could be used in the US. It’s just that it can only be used in the US with a hub on the 868.4 frequency.
In some other regions, it will be illegal to run a device of the US frequency because it might interfere with local first responder communications. But again, first responder communications run on different frequencies in different areas.
But their device cannot be used with a US model smartthings/Aeotec hub, because of the frequency mismatch.
What you would need to look for is if the device has an FCC license. If it does not, it is not legal to run in the United States regardless of the frequency. But if they did get an FCC license for it, then they are technically correct, that it can be used in United States.
The FCC license number should be printed on the device somewhere if they have one. It should also be printed in the user manual.
Just as an example, Gledopto is a company which sells ZigBee devices in many different regions. The Zigbee frequency is the same everywhere, that’s not an issue like it is with zwave, but you will notice that the FCC license is the second line printed on the back of the case. Without that license, it would not be legal to sell in the US.
So… does the device have an FCC license? If so, it is legal to operate in the United States. If it doesn’t, it isn’t. The Z wave alliance regional frequencies are recommendations, but their legality is left up to each individual country, and the US does not restrict the use of frequencies for other regions.