I would take it to mean if there's a delay in transit or you send the sample and it arrives on an off day (Cornell only does insulin testing on Monday, Wednesday and Thursday) you have less to worry about using a red or heparin tube.
As per the study:
Results: An overall effect of time, tube type and temperature was identified (P = .01, P = 0.001, and P = 0.001, respectively). Serum and heparinized samples had similar concentrations for 3 days at 20°C and 8 days at 4°C; however, after 3 days at 20°C, heparinized samples had significantly higher insulin concentrations (P = .004, P = .03, and P = .03 on consecutive days).
Conclusions and clinical importance: These results suggest an ideal protocol to determine insulin concentrations involves using serum or heparinized samples with analysis occurring within 3 days at 20°C or 8 days at 4°C.