Good morning,
The stated reason for a tube was
the time taken for tubless sealent to seal and my guess is that a latex tube may be closer to tubless than butyl one.
At first glance adding a tube might be seen as having no relative effect, but if you are looking for tiny differences who knows as it may be "the straw" that changes tyre performance on the test right.
40kg does sound reasonable but one very big difference between the test rig and the road is that the load is constant on rig and highly variable on the road. If you let enough air out of your tyres you can really feel them bounce as you pedal, so clearly the load is varying significantly.
Neither this test or BBR change the "road surface" which could reasonably be exected to give different results.
These tests remind me a bit of HiFi tests in the 1970s, when a perfect sine wave was fed in an amplifier and the amp's performance was measured on how much it distorted it. By this time nobody could hear the tiny levels of distortion.
In other words these tests boil to numbers of limited real world significance beyond, yes you can buy tyres that have different rolling resistance. Something anyone who has riden Schwalbe Marathon or Vittoria Randonneur already knows.
Bye
Ian