A 52-year-old man with CKD stage 4 (eGFR 22 mL/min/1.73m²) due to diabetic nephropathy presents with fatigue and dyspnea on exertion. Hemoglobin is 8.2 g/dL, MCV 78 fL, serum iron 45 µg/dL (normal 60–170), ferritin 320 ng/mL (normal 30–300), and TSAT 18% (normal 20–50%). Reticulocyte count is 0.8% (normal 0.5–2.5%). Serum creatinine is 3.8 mg/dL, calcium 7.8 mg/dL (corrected), phosphate 5.2 mg/dL, and iPTH 285 pg/mL (normal 15–65). What is the primary mechanism responsible for the anemia in this patient?