Step 5 is NOT "cut through the adhesive", that's steps 5-10. Step 5 is "Begin the careful process of cutting through the adhesive, starting at the right side where you already softened it. Proceed carefully, slowly, and warmly through the following steps."
It's extremely easy to crack the back glass when nearing and rounding the corners. It's probably a good idea to soften the adhesive with heat as you go.