It's the same failure mode as a Sybil attack, but it's called a 51% when there's the additional assumption of the hashrate being hard to obtain and evenly-enoug

It's the same failure mode as a Sybil attack, but it's called a 51% when there's the additional assumption of the hashrate being hard to obtain and evenly-enough distributed to mitigate sybiling, and that assumption is being violated.