I think your understanding is correct, although many would not consider that as acceptable support!
I found (and lost) a rather old comparison study of HD vs SSD and Time Machine use that indicated that the SSD ran out of write/erase sequences before the HD filled its disk with data. However, the SSD hardware was
built in 2003 
, hopefully a more recent study can be found.
A secondary problem was mentioned in other sites: An SSD requires an almost constant electrical supply to maintain its data (various methods allow that electrical state to be maintained during relatively short periods [ie; overnight]. But the problem can become catastrophic when the SSD "dies". Once the electrical power is lost, the drive is virtually a brick. A HD, on the other hand, can often have its magnetic data collected, even if other mechanical parts wear out (physical head crashing excepted, of course).
Other sites remarked that while SSD offer a very great improvement in Read/Write speeds, Time Machine itself is not very fast. And since most TM backups (after the initial backup) are done with quite small amounts of data and are quite fast. The user may never actually see or "feel" any speed difference by using an SSD. Therefore, the increased cost per bit of an SSD over a HD is wasted money.
Lastly, I have seen differing statements about whether or not Mojave automatically converts external drives to APFS; Disk Warrior says it does not, others say it does. This conversion becomes a problem when TM tries to write "hard links" which APFS does not support (except in its Beta versions).
If ones takes care to avoid that conversion and the drive is not encrypted, TM appears to be usable on either type hardware.
You are asking questions that are making me second guess the purchase of this iMac with only SSD internal storage!
But that speed is sure nice...
"Good luck young Will Robinson!"
"Live fast, die young?"
