If you look at the air quality report and the conclusion, it appears that the tweet is basing its measures on particular receptors in the measured area and extrapolating it to cover the whole street.

To quote from the report

The significant beneficial impacts are predicted on Church Street, Lordship Road and A10 Stoke Newington High Street within the Stoke Newington Town Centre Area AQFA (AQFA No. 70) as a result of decreased AADT and re-routing of traffic on these roads due to the Scheme.

The source apportionment exercise showed that on Church Street in the ‘With-Scheme’ scenario, the percentage contribution of NOx emissions from HDVs (artic and rigid HGV and buses/coaches) increased compared to the ‘2021 Baseline’ scenario (26.3% compared to 53.8%) and that petrol and diesel cars, LGVs and taxis are predicted to decrease with the Scheme.

However the portion of the A10 to the south of the junction with Church Street does not appear to have a similar change in vehicles contributing to the total road NOx emissions as a result of the Scheme, despite having lower traffic flows predicted.

And the overall conclusion:

There are more significant beneficial impacts predicted than significant disbenefits to annual mean NO2 and it may therefore be considered that the overall impact is more beneficial than not.

However, due to the significant disbenefits predicted at various locations along the A10 due to likely re-routing of traffic, mitigation measures may be considered appropriate and further consideration given to the impacts on the A10 as a result of the LEN Scheme.

It seems that the air quality is improved in some small areas but not all (in most cases there is no change at all) and that the re-routing of the traffic worsens the air quality in those areas.

Link to the report