No, she is using a blend of chest and head voice in that song. For a true musical theatre belt sound in a similar range to At The Ballet, look at Dying Ain't So Bad from Bonnie And Clyde, Rainbow High from Evita, The Wizard And I from Wicked, ect. You'll be able to find versions of women singing those songs who are belting those notes and your ear will hear a difference between that and what Kelli is doing in that song.