He didn't =/= He can't. Losing a battle is not same as being weaker. He was clearly more capable than Magellan which remains unchanged regardless of who won the battle. But Iva was clearly proved to be weaker than Magellan. So I don't get how you can even compare them. Losing a battle against Magellan is the only thing they have in common which means hardly anything while comparing their calibre.
Only to prove my point about Ivankov being underrated.
Blackbeard and crew would have
DIED if
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. Ivankov on the other hand
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and after being poinsoned was able to
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and also fight immediately when arriving at Marineford.
Clearly his resilience is far greater than Blackbeards entire crew, but this goes unnoticed. He also was able to take a attack straight to the face from akainu and got up and made a gag about it, only later passing out from exhaustion and intense heat.