Did they check the sizes of the effects over time since the protest? If BLM achieves their aims and gets the reforms they want, killings by police might remain reduced (but perhaps less so), while policing would become less politicized and scrutinized, so become more effective again.
EDIT: Towards the end of the Vox article:
The good news is that even if Campbell’s finding about the increase in murders following BLM protests holds up to further scrutiny, the effect doesn’t appear to last for long. By year four, Campbell no longer observes a statistically significant increase in murders, indicating that whatever is going on with murders is hopefully not long term.