If you are wondering if the theoretic accuracy of this simple and cheap SLM has any practical meaning, here are some measurements in comparison with IEC-61672-1, class 1, Brüel&Kjær 2250 sound level meter, courtesy of D-r Enrico Armelloni.  

The MEMS microphone used in the test is ICS-42432 (slightly older, and perhaps more accurate model), in protective shell which also acts as 1.27mm adapter, connected to ESP32 running the GitHub sketch. D-r Armelloni went to great length testing the MEMS+ESP32 setup, including various sound amplitudes, frequencies, pink and white noise, etc...  

The detailed calibrated results so far are in the excel file in the ‘Files’ section.

Please have in mind that this is probably the best-case scenario, i.e. it is not expected that every single piece of MEMS microphone will produce such close results. And if you wish to be confident in the measurements, you will need to do similar calibration on your setup. Also note that the range of the used MEMS microphones is about 35dB to 116dB and not suitable for i.e. low noise measurements.

What test does validate is the principle of how noise is calculated based on sampled sound from I2S microphone. On D-r Armelloni's advice, I also removed all misleading references to 'Fast' and 'Slow' in the project description, as the code never did any time-weighting on the sampled values. It only calculates LAeq values for various periods, which I believe is currently the most useful metric.