Giving up a car or going vegan is just as difficult to quantify as choosing to be childfree, for the simple fact that how much of an effect it has depends entirely on how much that person used the car in the first place and where their vegan friendly food is now coming from. I know I consume more avocado and chickpeas and other non-local foods than I ever did while eating meat.
And the article in the Guardian mentioned switching to an electric car, not going entirely car free.
You're right that the graph is only looking at CO2. But even just looking at the CO2 aspect would indicate it's nigh on impossible to cancel out the footprint of someone already in existence, considering all the smaller changes only add up to ~10tCO2e savings annually while each person is, on average, contributing close to 60tCO2e per year.
Even if if an individual choosing not to have children was ridiculously un-environmentally friendly, and we bump their contribution up to 70tCO2e, and a parent and child were both vegan from birth and environmentally conscious and only contributing about 50tCO2e annually each, they still contribute 30tCO2e more to the environment overall than the person who doesn't give a crap.
And that's a parent who, from a reproduction standpoint, is only replacing herself. When you take into the account the number of families who have 3+ children, and families that split up and have more children with new partners, and many of those children will grow up start their own families...it all quickly adds up.
And that's just the CO2/carbon footprint aspect.
I don't object to the statement that going vegan is one of the biggest things an individual can do. I agree with that. But I do object to the statement that it's the biggest thing an individual can do, because it's simply not true.