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Seems like from the blogpost that the writer and the founder were part of the same cohort at SpaceX. I would hazard a guess that they became friends, and had planned to do this together, and he joined ("hired") as soon as feasible for him (or the founder gained enough traction to pull him away from SpaceX).

[quartesixte]

Oh and as an additional comment: Propulsion Engineers are a niche, specialized branch of mechanical engineers. They are the ones who usually design engines.

The writer probably was referring to the fact that he was just a regular mechanical engineer and not a Propulsion Engineer.