I find that article to be badly written..
A main gearbox/main transmission isn't just "a part" ... they are a very expensive assembly / sub assembly/ being the heart and soul of the drive system of this helicopter.
Guessing they cost about a million (US $$) per copy. (Am I close?)
Overhaul and repair of such complex assemblies takes time.
If you look at the forecasts and long lead times for major assemblies like this, and then the usage rate increase that attended the 225's removal from service, this looks like a case of (1) bad planning (at a macro scale) and (2) bad program management. (Industry wide supply chain issues considered).