Why can tristana Jump out of Malphite ult?

She was in the cast time phase of the jump which cannot be interrupted. If Ult hit her after the jump started the jump would have been cancelled.

It's canceled the same way as pretty much every other non-unstoppable dash: immobilizing cc during the length of the dash.

Immobilizing cc during the CAST TIME for the dash does not work, just like all other mobility skills.