This (small) study showed a decrease in hypertensive disorders with conception before 26 months postpartum. That said, I agree with your docs in the case of HELLP - for complex reasons that I will try to explain sensibly.
All of these conditions seem to have different progressions and different triggers, but to share a common final pathway. It's beginning to look like some placental bed disorders (preeclampsia with IUGR and late miscarriage when the placenta doesn't take over at the end of the first trimester) are downstream from a shallow implantation caused by an immune response to a foreign placenta, and some placental bed disorders (HELLP or preeclampsia with a fetus that is large for dates) are downstream from an immune response to a foreign placenta that's demanding too much from the mother's metabolism. In either case you get a very annoyed maternal immune system and a lot of other symptoms in common.
So if this holds up: if you had HELLP, and a decent-sized kiddo, you *don't* want to conceive until some of the trophoblast that remodeled the spiral arteries supplying the uterus have died off, postpartum. But if you had PE with a small kiddo, you *do* want to conceive before that trophoblast has died off.
It's all a targeting problem to implant the placenta Just Right, not too hot or too cold. (Do you feel like Mama Bear yet?)