Why does the manual for the inner tube say the maximum pressure is 4.5 PSI? I recently picked up a pair of Schwalbe Aerothan inner tubes which are made of plastic and very lightweight.  The manual for these tubes indicates that the user should not inflate past 4.5 PSI. Quote:

ASSEMBLY INSTRUCTIONS AND INSTRUCTIONS FOR USE
[...]
Always use a suitable rim tape.
Inflate Aerothan Tube before assembly. Max. 0.3 bar/4.5 PSI. Inflating
to a high air pressure when not mounted may cause deformation and
damage to the tube.
Note: Aerothan Tube should not wrinkle.

My question is, how can this be possible?  Surely I need to be able to pump the inner tube at least up to the tire recommended pressure, which is 90-120 PSI.
Am I missing something important here?  I understand from other posts like Does the inner tube affect the maximum PSI? that inner tube pressure should not matter, but surely if the manufacturer warns not to go above 4.5 PSI, there must be something I am missing?
Most likely this low number refers to inflating the tubes without the tire, as a balloon.
When installed inside a tire, the majority of air pressure is held by the tire casing, not by the tube.
A similar warning about not inflating outside of/without a tire came with my Tubolito inner tubes, but the wording was more clear in my case.