I feel this is very oxygen centric thinking.
Life on earth existed for ~1.2 Billion Years before the Great Oxidation Event seeded Earth's oceans and atmosphere with O2.
That is 1/3 of the entire time life has existed on earth.
Yes, finding a very reactive element like oxygen in the atmosphere is a great indicator of an ongoing organic process on an Exoplanet.
But in our sample size of one we already know that No free oxygen does not mean, no life. And while our sample size of one indicates that intelligent life comes after o2 in the atmosphere we have no idea if this is always the case.