It looks like Guardian's quiet series that runs on nat gas now starts at 22KW. But they also have a diesel residential serios that starts at 15KW. So, you're right, it's possible Smarty has one of those.
But I wouldn't buy a Generac anything after looking at the ratings and stories at Amazon for their basic whole house nat gas generators. Last time I looked, they had a lot of 5 star ratings and a lot of 1 star and not much in between. The 1 star were people claiming to have units that showed up inoperable, failed in the first few months, failed right after the 2 year warranty was up, etc. And so many of those with problems were saying that go poor customer service from Generac. You expect to see some complaints on any products, but what was there on Amazon looked unusually bad to me.
The 10kw gensets had a Turkish Fiat 4