I'm not an expert, but as a lay person I agree with you.
However, I'm guessing people running these experiments are just looking at all the options. Hydroponics is pretty much a solved solution, no? I suppose it makes sense to run these experiments to prove what we already know. I suppose there is a hyper slim chance that lunar soil was somehow beneficial for the plant and made growing easier.
I think the nitrogen issue is probably secondary to the water issue. Hydroponics use way less water. Water is heavy and expensive to ship to the moon. Anything to reduce weight will probably be the priority.
I don't see how shipping more soil and more water would be better than hydroponics, but sometimes we write papers for problems we don't have yet, but might have in the future. Some moon colony might be struggling for physical space and need to use soil to grow supplies in an emergency? Might be nice to know they can reserve some moon land and follow these guides to produce food in the soil.
My wife got her PhD in Physics and one of her favorite anecdotes is the critical piece of technology that made her project possible was derived from a mathematical paper written 200 years ago. The dude didn't have the technology or science to know where his math would be useful, but he published it anyways. Eventually, someone needed it and found it useful.