The warning is near the beginning of the manual. Later, when discussing shower and tub connections on page 28, the manual says:
Caution: Do not use Uponor AquaPEX piping to connect the tub and shower valve
to the tub downspout as this may create excessive back pressure in the valve,
causing it to remain slightly open.
if in fact that statement is made by Uponor, it is in contradiction to a simple google search of pex outflow copper which returns this as one example
Does PEX have the same flow as copper?
Due to the minor differences in the tub/shower valves, the flow rate at the Test
Fixture (TF) was slightly different between the PEX and copper systems. The PEX TF > hot flow rate was measured at 1.7 gpm, while the copper TF hot flow rate was 1.5 gpm
https://www.homeinnovation.com/-/media/Files/Reports/pex_copper_pressure.pdf page 7 of 11
I have always known pex to outflow copper... 3/8 pex outflows 1/2 copper.
The statement of pex creating back pressure between valve and open spout is idiotic.
I would not use a straight length of pex, which would be about 4-5 feet or less, between the valve and tub spouts because of thermal expansion. Something like 1" per 100' per 10°, that's why you run expansion loops for runs of pex. Having pex from shower valve to spouts, behind the tub/bath walls not easily accessible, if expansion forces from 50°F to 120°F temperatures everyday eventually pop it off will create a major headache, worse than the pvc cloud in ohio currently. For this one scenario using copper done properly between valve and upper & lower spouts would be better.