A 72-year-old Indian man with a history of subtotal gastrectomy 8 years ago for peptic ulcer disease presents with progressive weakness and cognitive decline over 6 months. His wife reports he has become forgetful and unsteady while walking. On examination, he has glossitis, loss of vibration sense in both lower limbs, and a positive Lhermitte sign. Laboratory investigations show hemoglobin 6.8 g/dL, MCV 132 fL, WBC 2.8 × 10⁹/L, and platelet count 95 × 10⁹/L. Peripheral blood smear shows hypersegmented neutrophils, macro-ovalocytes, and occasional nucleated RBCs. Serum B12 is 65 pg/mL and serum folate is 8.2 ng/mL (normal: >5.4). What is the primary mechanism of anemia in this patient?