So METAR measured at a certain time point in the history somewhere around the airport is more reliable than the actual measurement of temperature at the exact time, at the exact location where the engines are?
Go with what the aircraft says. The performance calculations cover you when you would eventually end up using engine anti-ice, based on what the aircraft is telling you.
Turn it the other way around. If METAR says 11°C, and temperatures drop 1°C... you're not going to use engine anti-ice because METAR says so?
(ps: TAT on the ground is same as SAT as long as airspeed is 0)