Is there any protocols to identify what sort of document MS-408 is?

[ololololo]

Can we say that a low H0 is not itself evidence of low information content or of non-language?

Well... A low h0 score is the norm for all languages. A high score indicates that the sentence is meaningless.
The English language score is approximately 0.6-1 bit per letter

As I understand it, H0 isn't some standard linguistic or grammatical metric used to judge whether a sentence actually makes sense. 
Since H0 is essentialy just a count of how many symbols exist in an alphabet, it tells us absolutely nothing about grammar, meaning, or structural well-formedness.
Natural human languages actually build in a ton of structural redundancy to prevent communication breakdowns—unless, of course, someone is intentionally trying to be vague. 

Chomsky’s famous "Colorless green ideas sleep furiously" is the classic, ultimate reminder of this: syntax (grammar) operates completely independently of semantics (meaning).

Yes, you're absolutely right  The indicator really depends on the number of letters in the alphabet.