You don't "understand correctly" (Whether honestly or from those that suffer from an the inability of Trumpers to recognize ANYthing that doesn't fit the Trump narrative, I can't say.)
Canada is a DUAL CITIZENSHIP country. Ergo, when he was born, he held DUAL citizenship.
He didn't 'become" a US citizen in 2011 - but gave up his Canadian citizenship.
It's really quite simple, for those who can lay their blinders aside for a moment.
Anyone who thinks a natural born citizen can have dual citizenship misses the point if natural born citizen. Quite simply you cannot be both a dual citizen and a natural born cotizen of the United States.