It’s not so much power is lost through the wheel/roller interface as the reading comes from the crank.
training peaks explain it thus
On a rear wheel resistance trainer, there is little to no momentum of the rear wheel. If you stop pedaling the rear wheel comes to an almost immediate stop. Because there is resistance around the entire pedal circle, your legs are not used to having to produce power throughout the entire pedal stroke. As a result of this inefficiency, more strain is put on your cardiovascular system. As a result, this reduces your ability to create the same wattages as outdoors.
To be clear, it isn’t just the momentum (stored kinetic energy) of the rear wheel that keeps us moving out on the road, but rather that of our body plus the bicycle as a whole. An easy way of envisioning this is to imagine trying to stop the rear wheel of your bike wheel when it’s spinning rapidly in the workstand by grabbing the tire with your hand versus trying to stop that wheel while spinning out on the road using the same technique. In general, until you are “trained” to ride on a rear wheel resistance trainer, your watts will be 20 to 30 watts lower on the trainer than outside.
They claim that the inertia with a direct drive trainer solves this issue.. But would be useful to have real world experience
As you say its pretty irrelevant, but still worth considering when in market for a turbo if you are not relying on virtual power.