No, that's not what the site said. It said nothing about Old English being "closely related to German and Icelandic". It said that the *grammar* of Old English was much closer to that of German and Icelandic.Old English is closely related to German and Icelandic, more than the modern English. This was clearly stated on the presented site
It still does not say that modern speakers of either German or Icelandic would be able to understand Old English. The changes in both vocabulary and pronunciation would be the problem.