Not true. I ran APFS on my 2013 27” iMac with a Fusion drive and Mojave with no issues at all. If/when you move to Catalina or later APFS becomes mandatory in fact. When someone has trouble they tend to immediately blame whatever they did last as the cause. There is nothing magical about a Fusion drive. They are simply an HD and a SSD combined in software to appear to the system as a single drive.
Now wait for someone else to come along and warn that their system blew up in their face when they formatted to APFS. You already know that’s not true since you are successfully using APFS with your Fusion drive.