Two points remain unresolved in the published data on the phosphonate-containing GMA–MMA–OEDP ion exchanger: the calculation of chemical stability and the explanation of Cr(VI) removal by an acidic cation exchanger. This study re-examines the reported synthesis, FTIR data, static exchange capacity, and Cr(VI) and Pb(II) sorption results, and compares the recalculated values with recent phosphonate-based materials. The most rational synthesis conditions were a copolymer:OEDP mass ratio of 1.0:1.0, 90 °C, and 10 h, yielding 80% product with a static exchange capacity of 5.92 mmol-eq g⁻¹. Recalculation showed that 87.67–90.20% of the initial capacity was retained after treatment with H₂O₂, HNO₃, NaOH, and H₂SO₄; the actual loss was 9.80–12.33%, rather than less than 7% as previously reported. The published Pb(II) capacity was 40.4 mg g⁻¹, with 97% removal and an equilibrium time of 8 h. For Cr(VI), the reported values were 40.3 mg Cr g⁻¹, 96% removal, and 2.5 h to equilibrium. Pb(II) binding is consistent with proton exchange and coordination at phosphonate oxygen sites. Cr(VI) behaves differently: at pH 3.7–3.8, chromium is present mainly as anionic species, so conventional cation exchange cannot account for the result. Mechanistic verification requires separate determination of Cr(VI) and total Cr, XPS analysis, redox-potential monitoring, and surface-charge measurements. The reassessment corrects the numerical error and separates experimentally supported findings from hypotheses that still lack adequate controls.