To be broad in the beam means to have wide hips or large buttocks, and this sentence describes a woman who has wide hips but is skilled at hiding it.
Context: "Ms. Lampton was maybe forty, with wide hips and a large buttocks but camouflaging it very skillfully."
Hypothesis: Ms. Lampton was maybe forty, broad in the beam but camouflaging it very skillfully.
Based on the context, is the hypothesis true, false, or unknown?