Data is a value obtained from direct measurement or computed in a known way.
For example, you can measure the length, width, and height of a box - these are direct measurements. You can also calculate the volume using those measurements. Any "data" that is not directly measured or directly calculable is (technically) "fake".
What is happening in this case is that the MEGS-A sensor (which measures EUV between 5 and 30 nm) is broken. However, the MEGS-B sensor (which measures UV between 30 and 105 nm) is working. NASA has created neural net that extrapolates the missing data from the MEGS-A sensor.
Consider the following: you have a machine which measures the dimensions of boxes. It uses one sensor to measure between 5 and 30 cm and another for dimensions between 30 and 105 cm. Imagine the 5 - 30 cm sensor becomes defective, but the engineers create a neural net that is trained on past data sets and can "fill in" values in the 5 - 30 cm range based on the data the 30-105 cm sensor returns - this is "fake" data. The "fake" data may be good or it may be poor, but it is not "real" because it was not measured.