The details may be more appropriate in the blog post. In fact, the paragraph about the one you quoted describes the issue in general terms:
3. Virtualizing ZFS
The ZFS file system used by TrueNAS combines the roles of RAID controller, volume manager, and file system all into a single software component. ZFS expects direct access to your disks in order to work properly, in order to issue direct SCSI or SATA commands and receive an expected and predictable response. The closer you can get TrueNAS to your storage hardware, the happier it is, and the better it can do its job of keeping your data safe.
Now people may want more details to understand the problem better. However, these words are reasonably clear:
ZFS expects direct access to your disks in order to work properly, in order to issue direct SCSI or SATA commands and receive an expected and predictable response.
Perhaps adding details to @HoneyBadger Resource post might be useful. But, then again, this particular thread has gone on for 32 replies. Meaning not everyone will be satisfied with some wording, and may always have questions.
For example, I wrote this Resource which covers some of the aspects of direct access to disks, (instead of hardware RAID…).
Yet people persisted in thinking a UPS was required for data safety. It is not. However, UPS(es) are a good thing for multiple reasons, some outlined in my Resource or following discussion.