I'm not sure that partial experiments which test none of the putative downsides are of much benefit to a debate though.
I certainly would contest, for example, the suggestion that Neil Armstrong walking on a sidewalk would have represented a moonwalking experiment, because we didn't need to experiment to learn that astronauts are capable of perambulation on earth or learn anything about low gravity from it. Similarly, if you wish to test the hypothesis that a welfare state would be best reorganized on the to each irrespective of his need principle with taxes adjusted accordingly, you're not creating evidence for it by learning that unusually poor people are better off after receiving one off windfall of $7500.
The assumption that poor people are better off with more money is already baked into the status quo solution.
I believe the person you're responding to is saying that increased taxation is such a necessary component of any UBI system that any test which doesn't take that into account is flawed.
Missions before Apollo 11 were not moon landing experiments because they didn't land on the moon.
Projects before ITER (hopes to be) are not nuclear fusion power experiments because the reaction is not self-sustaining.
Movie previews aren't audience reaction experiments if the audience didn't pay for the tickets.
Or are partial experiments to learn something in fact reasonable, normal and necessary?