Carry-on enforcement for United Airlines Basic Economy

I have a question about United Airlines Basic Economy ticket carry on bag policy. It says that

You're allowed a small personal item that fits under the seat in front of you, such as a shoulder bag, backpack, laptop bag or other small item that is 9 inches x 10 inches x 17 inches (22 cm x 25 cm x 43 cm) or less.

I have a laptop backpack that is larger than those dimensions (20 x 12.2 x 5.7) but fits under the seat. I won't have anything else with me. Will that qualify as a personal item or do I need to check-in the backpack? How strictly is the policy applied?