Totally correct.
HX Effects and Stomp both have 1 DSP core - Floor, Rack & LT each have 2 cores. Stomp has all of the same effect AND amp models as Floor, Rack & LT but has a limit of 6 blocks. HX Effects has no block limit but obviously has no amp models.
Loading a full Floor, Rack or LT preset into Stomp would produce a very incomplete result as you’d hit the block limit and most of the components of the preset would not be present. Even if the preset from Full Helix contained only 6 blocks, what happens if those 6 blocks are each incredibly DSP hungry? They’d run just fine on Full Helix as the processing can be split across its dual DSP cores, however on Stomp parts of the preset would be missing due to not being able to run fully on the single DSP core.
Loading a Full Helix preset into HX Effects would obviously incur the same errors re: DSP but with the added hindrance of not being able to include any amp or cab models present in the original patch. More errors.
All of the above means it’s more effort than its worth for Line 6 to make Full Helix (Floor, Rack & LT) preset files compatible with Stomp and HX Effects. We’d get error messages all over the place, incomplete signal paths which bore no resemblance to how the creator intended them to sound and there would be complaints all over the Internet. Much wiser to keep the file types separate, ensure platform stability and tell folks “if you want to copy a Full Helix patch into your HX Effects or Stomp, copy parameters over by hand and get creative within the routing and DSP limitations of the device!”