Tons of players excelled in their NCAA D+1 year as 18-year-olds. Some examples that come to MY mind include Cole Caufield, Matty Beniers, Matt Boldy, Cale Makar, Quinn Hughes, Trevor Zegras, Will Smith, and Gabriel Perreault, etc. I could go on forever.
Dean was a 6'7 214 lb physically mature monster dominating against high school children half his size thus giving the illusion he was talented. He's your typical bust giant prospect who winds up amounting to nothing because outside of their physique they lack the processing speed, the offensive upside, and the pace to play in the NHL. So unless he finds his niche as a high-energy grinder like Michael McCarron, playing more than 200 games in the league will be impossible.
As for E.J., he's completely useless offensively and I don't understand what the Rangers saw in this guy. Being a stay-at-home, shutdown defenseman shouldn't justify being a liability offensively, especially in today's game, which emphasizes skill and talent.