Smoke detectors in both regions are covered by special laws/codes different than other types of sensors. (They also don’t act as zwave repeaters even if hardwired.)
The smoke sensors were announced by Fibaro for the US market two years ago, but they have yet to receive Zwave US or FCC certification.
The US has some very specific audio signal requirements for smoke detectors, and while they are similar in principle, the details are not the same as those for Europe. This usually means a completely different engineering run, not just a frequency change.
BS 5839 recommends that a fire alarm system be sufficient to all persons intended. Sound pressure levels are generally recommended to be a minimum of 65 dBA and a maximum of 120 dBA. A reduction to 60 dBA is applicable in certain circumstances, and a sound differential of at least 5 dBA is only applicable in spaces with an ambient sound level greater than 60 dBA. Sleeping areas are recommended to have a sound pressure level of 75 dBA. The principles are similar to NFPA 72; however, NFPA 72 typically requires a 10 dBA difference, or 5 dBA above the maximum ambient noise, with a maximum sound pressure level of 110 dBA.
So I think it unlikely that the US and UK certified smoke detectors would be identical devices, although the Z wave commands might be the same.