I thought he did, I mean, Sabo was really strong for his age but he wasn't "super human" yet ... and I'm assuming a cannon used by world nobles would be stronger then any normal hand cannon(Sabo was just stronger then a normal adult, even Mike Tyson would die being blown up with a cannon).
Lastly, I think its clear because the story was set up after Ace's death.. it would had been no problem for Oda to make up that arc before the war, for example - after Alabasta the crew could had asked about Ace. So I really don't buy the explanation "Sabo couldn't had came during the war because he wasn't shown yet".
As I have said, its much more likely Sabo was just made to help
Luffy get over Aces death, and also it shown why Luffy cannot fail at becoming the PK - he was the youngest brother, last out to sea, and the last one to survive.. so he couldn't just stop after Ace's death he had to move forward.
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