No it's not to good to be true.
It's based on the Stirling engine, which has been around for more than a 100 years.
It is great technology, lasts a long time and is very quiet, BUT it is more expensive than traditional means.
As a rule Stirling applicatons tend to cost two to three times traditional solutions. So if a generator solution is $5K - $6K I would expect to pay $10K - $12K for a Stirling based solution.
From a physics standpoint, it is good, reliable, long proven technology.
Regretabley it is more expensive, primarily because of supply and demand i.e. scale. If we all started buying Stirling products the price would drop pretty fast.