There's also much more room for creating opportunity for one person without removing it for someone else.
In a business there's usually only one spot for a COO, but you can have many senior developers. Therefore the stakes are lower, because other members of staff don't see an opportunity vanishing when a promotion to senior dev is announced.
He's saying that a junior employee, who isn't experienced with corporate politics will take your advice to heart and that'll be the end of it. With the executive team, they're much more likely to use that advice as political ammunition, so you should be careful with what you tell them when they ask for more responsibility.