If you apply Occam’s razor, this explanation stands out.

I think there are other reasons and am capable of critical thought. A variety of synergies have been discussed publicly for at least 9 years. This article lists several of them.

In the sense that you can claim two things are synergies by just saying they are, sure.

Whereas in reality automotive companies do not organically have rocketry divisions.

Chrysler (now Stellantis), Honda, Toyota and Geely all had/have rocketry divisions and many more auto companies have/are involved in the Aerospace industry more generally.

If anyone thinks there's only one reason, it's a clear sign they're incapable of critical thought.