You have to peal the coversheet of, on both sides, and place it directly on top of the SSD after the installation, and make sure, that the Metalplate inside the Top of the case, that you screw back onto the case, gets in contact to this gel, to get a good heat spread towards the case. This panel is made of a kind of a gel, that transports the heat. The Case came with some kind of a “rubber panel”, a lot of people in the installation videos are throwing away.Then you could use TimeMachine, CCC or whatever your Backup system is, to play the Backup back, and you are ready to go. There are a lot of descriptions how to do the replacement out there, and the one I used are in German, so you have to have a look for yourself, but it is pretty much just put the SSD into the Case, plug it in, and do what the System required to use it as the new Startup Drive. Or you could replace the internal Drive by an external Thunderbolt3-SSD.Both has the risk to damage the Display, with ending up in replacing the rather expensive display panel, too. You could replace the internal FusionDrive, by yourself, or with a ServicePartner of Apple.You could now separate the two drives with DiskUtility, use the internal SSD for the OS, and place your Data onto an external HDD.While it is unusual, that the SSD-Part of the FusionDrive is still fine, and the HDD failed, it is for sure not impossible.

In your Situation it is something different. The linked article is misleading on that, as it seems that the approach of this article just was, to replace an internal HDD by an external SSD for SpeedGain, and therefore just not looked at the iMacs with a FusionDrive who already have the SSD Build in, so there is no Speed Gain available, with that replacement. That would be the same system like my one.

2017 imac hard drive replacement