The historical argument best supported by the primary source excerpt is: The Mongols were affected by the bubonic plague prior to Europeans.
This is evident from the description of the Tartar (Mongol) army suffering from a disease that caused significant mortality, suggesting that the plague was present and impactful before it reached European populations. The text discusses the symptoms and the impact of the disease on the Mongol forces, indicating they were already experiencing a plague-like situation before it spread to Christians in the besieged city.