a leading authority pointing out that English has sounds more gutteral than any in German.

Not true, by my understanding of it. Guttural is no longer used as a technical term in linguistics, but if it refers to any sound made in the throat or back of the mouth, German has a lot more than English.

Both languages have [ h ].

German has [ x ] as in nach, doch.

German also has the glottal stop [ ? ] before any word beginning with a vowel, as in eins = [ ?ains ].

Added. Also German has a guttural R, a uvular approximant to be exact.

Further, German speakers typically have greater throat constriction; I think their vowels are pharyngalized (the root of the tongue presses closer into the pharynx as a secondary articulation), though I won't swear as to the exact nature of this articulation.

I agree that 'guttural' is usually bandied about as a meaningless derogatory term, but I think in this case it's accurate in its literal meaning.