Well, it ain't that easy as it seems to be. Remember, the guarantee is about your money and not the score ultimately, no one can guarantee you a specific score, you'll have to earn it. you may get your money back but that's it. Besides, the starting point to be eligible for the guarantee is 620, basically, all they are saying is it's an 80 points score improvement guarantee which you can get from a lot of other online courses for a much cheaper price. In the end, it all depends on you and your skillset and how further you can take it. Any score is achievable on GMAT. it's a standardized test after all.
This is straight from Princeton review: At the beginning of your GMAT 700+ course, we’ll establish a baseline starting score. Your starting score can be either:
Your official GMAT score that you received no more than 1 year before the start of your course and that you submit to us before your second class session; or
The score you receive on GMAT practice test #1 in the Online Student Portal, which must be your first practice test in the course and which must be taken before the second class session.
You need to take the official test on or before the recommended test date for your course that we posted on our website at the time of your enrollment. If you registered for an official test on the recommended test date but it was canceled, you need to take the next official test that is available. If your official GMAT score did not increase by at least 80 points (if your starting score was under 620) or did not equal or exceed 700 (if your starting score was 620 or higher), then subject to the paragraphs below we will refund your tuition, less the fee for course materials (as reflected in your invoice) that we shipped to you and shipping charges.
In order to be eligible for a refund, before your test date you must:
Attend all scheduled class sessions.
Complete practice tests #1-#7 in the Online Student Portal in accordance with the schedule provided in the Online Student Portal.
Complete the 4 hours of private tutoring included in your program before your official test date.
Complete all assigned homework according to the schedule in your syllabus.
You must submit your official GMAT score as well as your guarantee request form within 60 days after your official test date.
*The relevant “score” is the total or composite score over all sections of the exam. For clarity, scores for individual sections from different exams may not be combined to establish the total or composite score.