If I am accepted into the PhD prep track, does this mean my acceptance is guaranteed into the PhD program?

No. All acceptances given while in the MS, PhD prep track are conditional. This means your acceptance can be withdrawn at any time due to poor academic performance, behavior, and other various reasons while in the MS program. Conditional acceptances are not guaranteed to be held for students that are 

not able to start the PhD program immediately after completing the MS program. If your acceptance is withdrawn from the PhD program, this is a PhD program decision, the MS program cannot assist with getting that conditional acceptance back.