Neither tag has a description, so it's not surprising they get used inaccurately.
I would have thought they were different, but if we cannot define them to be substantially different, then yes, they should be merged.
Suggested definitions (improvements invited)
Diet definitions applicable to cycling would be
- the kind and amount of food prescribed for a person for a special reason (health, weight, performance, etc)
Nutrition definitions can be distinguished from diet as
- The science that deals with nutrients and nutrition