No, it's not the most important.

The most important was clearly obtaining the PLC zero days to infect the physical machines. It's unclear to me why you choose to be so explicitly obtuse but in any case, for your own personal edification, feel free to read some details on how it went down -

[0] https://www.wikiwand.com/en/Stuxnet

[1] https://www.wired.com/2014/11/countdown-to-zero-day-stuxnet/

[1] https://www.hsdl.org/?view&did=792239

Zero day is worth nothing if you don't have someone to plug it in, but sure, be pissy about it.

This was a complicated operation that had many difficult steps. If any of these steps would not have worked, the entire project would have failed. Just pointing at one of these step as the most important does not show much appreciation of the other steps.

Sorry but Hum-int/ops is still the most important factor...especially with systems who has an air-gap.