This is what we call, " hasty conclusion or generalisation ", or simply, " fallacy".<quoted text>
You didn't provide any evidence. The fact that Old English was closer grammatically to German and Icelandic than to Modern English is not evidence that modern Germans can understand Old English.
Where is your evidence that modern Germans can understand Old English? We're still waiting.
You are wrong, modern German speakers, do have an idea on old English than modern English. Many people do know this.