I see your point, but still, had it been a reading comprehension test I was grading, the red pen would have come out for missing the meaning of "as he died" .
What did Guilliman do? He died. Not "he was dying".
As a sidenote, isn't it a bit weird that people are so quick to misinterpret this, so as to posing a possibility of Guilliman's return, considering the Ultramarines usually being so hated on the interwebs?