What year is the iMac? That can make a difference in what is going on.
And this is not an Apple refurb. Although these are the English-language forums and I don't read much French, your screenshot from System information shows that, probably during the refurb process, the mechanical part of the Fusion drive was replaced with an aftermarket SSD ( Samsung 860 EVO 1TB). Apple would never use a branded third-party drive.
Apple's Fusion uses two separate devices—a small blade-type SSD socketed to the logic board, and a larger mechanical hard drive connected by cable and residing in a traditional drive bay.
To make one Fusion drive, Apple's links the two via software. It appears the software link survived the aftermarket install of the Samsung SSD.
As long as the Samsung is working, this should be a reasonably fast arrangement. I prefer Crucial and OWC SSD's over Samsung but yours will work. It will not be as fast as a factory SSD because the 860EVO is limited to about 600MB/sec transfer speeds but a blade SSD can do 2800MB/sec.
For the standpoint of reliability, I worry more about someone other than Apple going inside. Were the original Fusion drive working properly before, it was potentially faster than putting a 6GB/sec SSD in the main drive bay based on drive performance reports I've collected.
Best practice for aftermarket conversion to pure SSD is to replace Apple's small blade SSD with a larger one, then "breaking" the Fusion link and leaving the mechanical hard drive for extra storage.
The only way we can know if your combo is working at reasonable speed is for you to get a utility like Blackmagic Disk Speed Test (here in the Mac App Store) and post both the read and write speeds. That may reveal that the conversion is delivering good performance.