Dieticians are doctors but only in the sense of being a doctor in the nutritionist field and not a doctor in other fields like your general family doctor or pediatrician would be.
Normally dietitians have at least a bachelor's degree.
If the dietitian have not received any doctoral level degrees such as a PhD (i.e. Doctor of Philosophy) or a professional doctorate degree, then they don't have the "doctor" title.
However some doctors may also have their degree and training in being a dietician so some doctors may also be dieticians as well.
A licensed nutritionist is a health care professional specializing in nutrition and dietetics who has received credentials from a nationally recognized nutrition licensing organization.
While a nutritionist is not a doctor, it is feasible that a doctor could also be a licensed nutritionist.
A dietitian is an expert in identifying and treating disease-related malnutrition and in conducting medical nutrition therapy, for example designing an enteral tube feeding regimen or mitigating the effects of cancer cachexia.
Many dietitians work in hospitals and usually see specific patients where a nutritional assessment and intervention has been requested by a doctor or nurse, for example if a patient has lost their ability to swallow or requires artificial nutrition due to intestinal failure.
Dietitians are the only regulated healthcare professionals licensed to assess, diagnose, and treat such problems.
In the United Kingdom, dietitian is a 'protected title', meaning identifying yourself as a dietitian without appropriate education and registration is prohibited by law.