Master's Program at Harvard Extension School

[alexf.1990]

(nobody would ever describe the HES Data Science program as a MS in Data Science, because it would be inaccurate to say so)

please expand              
in what way is it inaccurate

Because MS in Data Science means something very specific - that the school awarded you a Master of Science degree with a major in Data Science. The HES degree is not a Master of Science degree, for one thing, and your major is Extended Studies - your concentration is in data science.

It'd be like describing a TESU BALS with a psychology concentration as a Bachelor of Science in Psychology

I disagree with this. Firstly, Harvard doesn't use the term "major" for any of their degrees. The College uses "Fields of Concentration" to describe their majors.
[/url]. The Extension School uses the term [url=https://www.extension.harvard.edu/academics/graduate-degrees]"Fields of Study" to describe its different degree focuses.

Secondly, your analogy is a bit off. A BALS from TESU is fundamentally different than a BS in Psych because it's more focused on Liberal Studies. The same isn't true of degrees earned from HES. "Extension Studies" does nothing to describe the subjects actually covered in the program. The only reason that language is in the degree is to differentiate it from the on-campus degree. The language is there purely to appease the other Schools. This is abundantly clear when you look at how Harvard wants you to list the degree on your resume:

• Bachelor [or Master] of Liberal Arts, Harvard University Extension School.
  • Include field of study, minor, and degree honors when applicable. 
    
• Bachelor [or Master] of Liberal Arts, Extension Studies, Harvard University.
  • Include field of study, minor, and degree honors when applicable.
    

So long as you note that you earned the degree from Harvard University Extension School, you're free to leave off the "in Extension Studies" portion of the degree.