Because the third trimester is the politically expedient argument. It's a lot easier to frame abortion as killing babies when you talk about the times where it happens at later stages of viability, rather than the overwhelming majority of cases where it's earlier and less questionable all-around for anyone who isn't a zealot on the matter. He'll likely quote statistics at you like there being 10k+ "late term" abortions per year in the US, but last time I bothered to contest that number he basically picked and chose it based on a very select source (given that there's nowhere near a consensus as to what a late-term abortion is amongst professionals) and ignored the actual reasons why those happened, preferring to believe the fiction you present that a number of doctors are chomping at the bits to rip babies out of their mothers and only the wise guiding hand of the State stops that from happening. Context as to why they happen at those stages is irrelevant to him and his argument; that they happen is bad and means accessible abortion is bad and accessible abortion must be bad because it's a Democrat position that it is good.
Approach his post from the viewpoint of an extraordinarily partisan person. Then it starts to make sense, in its own way.