"5 6" is a string, not an expression. If you convert it to an expression with ToExpression, it will become 30:
In[15]:= ToExpression["5 6"]
Out[15]= 30
Mathematica does not calculate anything inside a string.
In your original question, "\\text{H}_2\\text{O}" is a string. but the output is not a string:
In[16]:= ToExpression["\\text{H}_2\\text{O}", TeXForm] // StringQ
Out[16]= False
To my great fury, some years ago the standard output view stopped showing the "" quotes, and this leads to amiguity and misunderstanding.
