The protein they're trying to manufacture is indeed quite simple - AFAIU both BioNTech and Moderna put together their sequences in a weekend. (Though there was a more involved process of winnowing down the sequences for the most effective ones.)

The technically challenging parts are:

- delivery mechanism: you need to take a very unstable molecule, protect it from the environment - both external, and when inside the patient - and insert it into a human cell. (This is called the "platform", and is usually developed independently from the specific payload.)

- manufacturing: both producing the mRNA itself at a large scale, and inserting it into the delivery mechanism, at a large scale and in low-temperature conditions

- testing: the newly-developed payload and the existing platform were integrated at small scales within weeks, but testing the thing for safety and efficacy took months

EDIT: As schoen pointed out, this was not actually released by Moderna, but reverse engineered by third-party researchers. Original text was: "Hence they feel safe releasing this. Their moat is not the gene sequence, their moat is everything else."