The specimen used for the transmission electron microscope is placed in a low-temperature environment. In most cases, the temperature is lower than the Mf point. So, why the unstable retained austenite does not change to martensite?
Most likely, there is substantial time between forming martensite (quenching) and taking it to the low-temperature environment. This stabilises the retained austenite. Once it is stabilised, further exposure to low temperature is not known to cause a phase transformation.