tobirama done a sneak attack and madara killed his brother for EMS
finally the story showed the finish of the battle and now you can see the difference between Izuna vs Tobirama and this one.
1. the proper stab and still Madara survived http://www.mangareader.net/naruto/626/14
Tobirama didn t even stabbed Izuna, only scratched the left side http://www.mangareader.net/naruto/624/12
2. Hashirama after the battle http://www.mangareader.net/naruto/626/16 in next page he passed out
Tobirama didn t showed any sign on exhaustion or fatigue or injuries http://www.mangareader.net/naruto/624/13
so it seems i was right, izuna couldn t die by that injury, madara lied to them but in reality he killed his brother for the eyes since he realized he is no match for hashirama.
Tobirama used a sneak attack to injure Izuna, a proper fight would have been way longer, and the winner would have been injured and very exhausted since they were considered equal since kids.