> do the actual treatment, don't mess around with diet
No, you should do both. Telling people not to improve their diets is just as irresponsible as telling people not to take an effective medication. Especially in cases where there aren't effective treatments available, like some autoimmune and neurodegenerative disorders.
This comes with all the obvious disclaimers like do your own research, make sure there is at least some evidence for the diet's overall healthiness and interactions so that it can't make things worse.
Personally, I effectively cured a 2-year stint with long covid through diet (essentially a low sugar paleo diet). Within a few months I was able to exercise again without a fight or flight reaction after years of no improvement. Doctors (including one who was also battling long covid) told me there was nothing they could do to help, but they hoped that some of the experimental treatments like stellate ganglion block might eventually bear fruit. I could have waited around with a much lower quality of life, but instead I started the very difficult diet, and it paid off. There's plenty of snake oil out there, but don't throw the baby out with the bathwater. The world isn't black and white.
> very effective treatment which is mildly uncomfortable and time-consuming.
I'm not certain, but it seems like you are implying that diet is the easier choice. This is almost never the case, broadly speaking. Perhaps it's different in your specific community. The difficulty of sticking to a diet and knowing what foods should actually be included and excluded from a diet is probably the most significant hurdle to treating conditions that could benefit from diet (especially early onset conditions related to overall health). This is mostly a consequence of what's available and affordable in America, but is nonetheless a significant setback for recommending diets.
But where a good diets truly shine is in prevention. Just due to how unhealthy the modern American diet is, it's pretty easy to find benefit, so that should be a starting point alongside exercise when you don't have a cure-in-a-bottle handy.