I gave you the exact calculation the way one has to do it when computing with irrational numbers and it gave 12, and even justified the steps with things that are taught in all high-school algebra classes. Your belief or lack thereof is irrelevant to the fact of the matter, which is that without specifying a shape for the blanket the problem is ill-posed, even specifying that the blanket is rectangular the problem remains ill-posed unless one also specifies that the side lengths are rational numbers of units (or better for simplicity integer numbers of units).
Applying that logic would throw out virtually every story problem in every grade school textbook. It’s an easy problem with an easy solution (at least for an adult), and does a good job of introducing concepts of area and perimeter to a second grader. She can deal with rational and irrational numbers when she’s 18 or 19. And maybe those writers should be more specific. For now, this is a fine problem for a child to solve.