Yet that is not what was stated. The risk of pregnancy is not zero. If we really want to get into it, abortions are not 100% effective either. So yeah, extremely unlikely, but still a possibility that one has a child.

You are technically correct, the best kind of correct.

The context of the conversation, however, was that people had children as a non intentional byproduct of having sex, which presumably people had more of due to lack of other entertainment options.

To which I thought it was noteworthy to point out that children are effectively no longer a byproduct of just having sex, especially for the demographics in which birthrates are declining. Hence, regardless of the amount of sex people are having, I would expect the birthrate to still decline.

That does make sense. It's just dangerous to say the chance of getting pregnant is zero, since that could influence someone's decision/understanding and lead to an undesired shock of an outcome.