This is sentence comes from beat the gmat flash cards.

This is the formula : One of the (Plural noun) that/who [Plural verb]

Example:
He is One of the persons who bake spectacular cakes.

Persons? Isn't more than one person considered People?



Another sentence:

He is the only one of the employee who was promoted.

Isn't this considered wordy and redundant?

Can't you just say, He is the only employee who/that was promoted?