This is absolutely incorrect. We know way more about good diet and the effect of nutrition than we used to. We do not know everything, but claiming that nutrition science is not useful is horribly wrong and misleading.
If you are offering diet advice, you should absolutely make use of all that we do know: and that knowledge comes from nutrition science.
If our end goal is to offer everyday guides on what foods to eat, nutrition science has a really hard time offering anything useful.
The predictions are necessarily minutely targeted at particular nutrients in isolated trials. It's really not possible to come up with a diet plan that replicates those isolated trials. I guess what I'm saying is that we don't (currently) have the capabilities to run studies big enough to examine the whole system in all its intricate pieces.
You can do surveys - and I think they are probably the most broadly-useful current nutrition studies, because they give recommendations like "food-related disease is significantly lower in people who eat a mediterranean diet". That's an actionable prediction.
As another counter-point to my argument, isolated experiments seem to work well in providing recommendations like suggesting to stay away from carcinogenic foods, etc...
If you are offering diet advice, you should absolutely make use of all that we do know: and that knowledge comes from nutrition science.