It is possible to have fuel dilution and not have a low flash point.
While there are more precise methods for measuring fuel dilution (e.g., steam distillation for gasoline and gas chromatography for diesel and gasoline fuels), the flash point is very useful as a pass/fail screening tool that is adequate for most used oil analysis applications. Because of the low flash points of most fuels, a sudden drop in flash temperature in a crankcase oil can usually be relied upon as an indication of dilution. However, there are exceptions. Because there is often some overlap of the light-end volatile constituents of some lube oils with the heavy ends of the fuel, the presence of fuel dilution may be less distinct. Another influencing factor is that hot running crankcase temperatures alone are often sufficient to boil off light-end fuel fractions, leaving the less volatile and more viscous heavy-ends mixed with the oil and potentially undetectable with the flash point test. Fuel dilution reduces viscosity of a lubricant.