There's an enormous drop in edit: late 2019, and the second drop starts in 2023.
https://www.sanfranciscopolice.org/your-sfpd/policies/depart...
> Starting on March 19, 2024, Flock Safety began installing ALPR cameras in various strategic locations across San Francisco. This rollout is expected to take place over the next 90 days. Per 19B ALPR policy, the administration of the Flock ALPR system is the responsibility of the Investigations Bureau.
How did the Flock cameras cause two crime drops before their installation?
The article's note about 2018 is talking about extending backwards, not forwards. It's entirely accurate, and a direct quote from your link.
The chart is trending down by January 2020, changes directions (upwards) right around the March 2020 spot, and again around (down) the July 2023 spot.
The fact that they only have data going back to 2018 means it's hard to say if the pre-COVID stuff was the norm or unusual.
To be super-clear, here's the chart annotated to show that 90 day window (black rectangle) in which the cameras were installed. https://imgur.com/a/i00Gna0
"that drop is obviously in early 2020", to reemphasize, is several years before the cameras got installed.