Link to source article here: https://royalsocietypublishing.org/doi/abs/10.1098/rspb.2020.2851 - unfortunately another journal with restricted access unless you're a member of an academic library or willing to pay (also another example of journalists lack of accuracy with science; BBC cite the source as 'the journal Proceedings B', which is of course Proceedings of the Royal Society (series) B). Can't find it on Research Gate.
Interesting stuff (if we could read the whole thing) - there is quite a lot of research on traffic noise, including a lot of papers from the Netherlands looking at poor breeding performance of willow warblers near roads, and a lot of research on shifts in song volume / frequency in response to traffic noise.
One possible flaw with using caged zebra finches is the fact that in the wild, a degree of habituation to noisy environments occurs, so the effects might not be so marked. On the other hand, lab experiments give some insight into possible mechanisms which might be responsible for observed effects in the field.