While usually true, in this case they used MIRI to measure the amount of missing heat when the planet was behind the star as opposed to next to it. A secondary eclipse instead of a transit. They can then make assumptions about the atmosphere with that measured temperature and the fact that it is a tidally locked planet.
remchien t1_jdya4ce wrote
Reply to comment by AlphaDrac in James Webb Space Telescope finds no atmosphere on Earth-like TRAPPIST-1 exoplanet by locus_towers
While usually true, in this case they used MIRI to measure the amount of missing heat when the planet was behind the star as opposed to next to it. A secondary eclipse instead of a transit. They can then make assumptions about the atmosphere with that measured temperature and the fact that it is a tidally locked planet.