First, great to see more research/deep thinking being done on repro history! The patterns between health/lifespan and repro history are quite complex, so seeing more focus on this area to tease out what's going on is good
Their interpregnancy interval (IPI) and recovery hypothesis is interesting and I look forward to seeing prospective studies on this to tease out the actual impact
That being said, I would not be surprised if the main modifiers that they called out for postpartum recovery (esp BMI, but also sleep and breasfeeding) explain most of this effect vs some "biological age" that they claim is accelerated
On the clocks side of things, some of the studies they used for reference here didn't control for blood cell composition - which they note is important - and the studies that do show pretty substantial changes across gestation. While they noted some studies that corrected for this, I would not be surprised if this explains another meaningful chunk of what the clocks are tracking
Also, pregnancy is a very unique physiological state and using "biological aging" clocks to try to pin aging to changes during gestation seems a bit contrived to me. They discuss clocks during pregnancy pretty heavily, but the longer term recovery postpartum idea and measuring that with these clocks seems plausible