What does this even mean? What is a developmental league? A league where players develop? Players develop in the NHL, the stats bear that out.
It's not a development league in the traditional sense of the term. Players do not develop from being not-NHL-caliber to NHL-caliber in the NHL. They adjust, yes. They improve, yes. But it's a high bar for entry, and players must have most of their shit together before they can step on the ice and find success. As opposed to lower leagues, where many players are not necessarily expected to be ready for that league when they arrive, but where there are mechanisms and systems in place to bring those players along and let them develop their game without being put in the pressbox every night. They get a lot more focused treatment from coaches in actual development leagues.
The NHL is not considered a development league, and any hockey person around pro hockey will tell you that. To be on an NHL roster, you have to be good enough to contribute. You are expected to have all the critical things in your game already in order. NHL teams don't spend the time or resources to develop a player on the fly.
Many people would probably be surprised at how little one-on-one coaching happens with rookies at the NHL level. There is
some, of course, but NHL teams don't mess around. There are very high expectations for young players and when kids require unexpected additional time from the coaching staff, that's when they get sent back down to the minors.