In order for the latter to be true, Magnus would have had to somehow sense Runes before the system descended, before his Will was truly and fully unlocked, and somehow through the mangled mess that was left of Earth's Runes.
While even after the Summoning began, Earth's Runes were in a mess, it couldn't be compared to how bad it was before the Summoning began.
It had to be remembered that Earth's Runes were in such a terrible ordeal because of the actions of others. They knew that Earth could produce a great deal of talents, and if the future generations were allowed to stack their understanding with that of past generations, they would grow to a level that—with the help of restrictions set by the system during the Summoning—it would be impossible to stop their rise.
