It is not helpful because comparing English from 1000 AD with Modern High German is the wrong premise to start off with.
I hear this premise repeated time and time again. Search the internet. I believed this premise, and actually started studying German again while waiting for my Old English textbook to arrive. It did not help.
I do not need to search the internet as I am fluent at German as well.
The knowledge of Modern High German helps little to none as far as the comprehension of Old English is concerned. From a modern German speaker's perspective, Old English – with a relatively small number of exceptions – is gibberish.
I hear this premise repeated time and time again. Search the internet. I believed this premise, and actually started studying German again while waiting for my Old English textbook to arrive. It did not help.