Depends on where you stand, we know that Gulliman and curze survive the heresy and are brought down in desperate incidents.If this is the lesson Russ sacrificed his sons and risked his life for, then he is the fool, not Angron.
Were Night Haunter and Guilliman protected by plot armour or were the Wolves/Alpha Legion?
We have extensive evidence showing us that Marines are nothing to Primarchs, we have one source where they suddenly become credible threats. Is it more likely that every prior source is exaggerating, or just this one?
Also keep in mind gullimans advisors words. One lucky shot to his head would have killed him.
Russ knew going in that the world eaters were dangerous he stated that they attacked many allies before this event.