I don't buy that as a factor. Given that the OEMs overprice the parts that they have to compete with aftermarket suppliers for, why would they NOT overprice the parts for which they have no such competition?

Note that being unable to profitably replicate the part does not necessarily mean that the part is naturally expensive, or that the OEM already sells it at a reasonable price. The part could just as well be hard to reverse engineer and QA for a third party, or the effort to do that might not be worth it given unknown level of demand, etc. OEM can actually have a natural price advantage over third parties in such complex parts.

As was just pointed out, making the part isn’t a large fraction of the cost of a replacement part. So, they need to both make the part and handle the full logistics chain to undercut prices.

Aftermarket suppliers have even more of a logistical burden than the OEM. They cater to a smaller market, and need to set up manufacturing for the part (which the OEM already has). Yet they still can sell the parts for substantially less.