This actually exacerbates the problem of lower birth rates. Longer life expectancy mean an even greater portion of the population is non-working and dependent on the shrinking share of working-age people. If people worked a greater portion of their lifespan that could compensate for lower birthrates but increasing retirement ages is extremely unpopular.

I would interpret "longevity and health" not just to mean increased life expectancy but to also mean increased time until you are infirm. This expands the percentage of your life that you produce more than you consume (i.e. 20 to when you become infirm for the average person), and decreases the percentage of your productivity that goes towards raising children (since you have more overall productivity but roughly the same amount goes to raising kids).

You can't cheat the math though. 2 people need to give birth to 2 kids to break even. (2.1 actually).

You can't, to be charitable I would assume that they are doing their math in kids/(person years) instead of kids/person.