According to the actual ordinance itself¹, according to a 2018 National Academy of Sciences, Engineering, and Medicine report, there's moderate evidence that e-cigarette use actually increases the frequency and intensity of cigarette smoking in the future.

Also, SF did in fact ban flavored tobacco products last year, but it seems that wasn't sufficient to combat the rise in e-cigarette usage among SF high school students.

In any case, it seems the ordinance doesn't actually ban e-cigarettes. It bans e-cigarettes that have not received FDA premarket approval, which is required by law for any new tobacco product. So basically, the city is saying the FDA isn't doing their job, and so the product that shouldn't be allowed to be sold in general without this approval is now banned in SF until it receives this approval.

¹https://sfgov.legistar.com/View.ashx?M=F&ID=7111897&GUID=7C3...