Going by the definition given in the wiki you’ve linked, a Sybil attack is about creating many fake identities to gain disproportionate influence in a network. A 51% attack in blockchain terms is specifically about controlling the majority of the network’s mining/staking power to override consensus.
Someone amassing 51% of the network would probably want to do so under some fake identities so others don't realize what's about to happen. Not the same, but probably related.
So I'd say they're not exactly the same.