why are you assuming an electric car will be an overall reduction in CO2?

It's not an assumption. Of course, it depends on a lot of things, but broadly speaking, they use less CO2. And the great thing about a carbon tax is that this shakes out through the system: if they're not reducing CO2, you would see it in costs and could react accordingly. This price signal is a lot more convenient than having to, as an end user, try to figure out what the best and worst things to do in terms of CO2.

Company will just hire the delivery drivers as contractors a la Uber and pass the costs onto them.

Same logic applies though. If you're a contractor and it costs too much because gas is expensive, you either don't do it or demand more money. Or maybe only people with low-emissions vehicles get into it.