I am not limiting Roy to anything and i think its possible he ends up a 30 goals, top sixer.
But, i am a statistical and data type of guys.
Concerning Roy, i will simply state a rule : The overwhelming big majority of top six NHL players and top 3 defenseman are in the NHL in their D+3. Big time junior players graduate immediately. NCAA/European stars sign after 1, 2 season, not 3.
I'm a statistical guy too, so I'll argue against this.
You're using a single stat / rule of thumb for what is a heavily multivariable process. Hockey players are not electrons and hockey teams are not alloys. There are a lot of variables, and so all rules of thumbs should be used with caution.
In the case of Roy I'll suggest the following reasons why he might
First of all he's a late developer. He wasn't drafted in the first round, and the team probably didn't plan on him being remotely ready at this time. If a first rounder had dominated the CHL for two years and Connor Bedard had called him the linemate that most impressed him, the team would have probably planned for him.
Second, it's an unusual situation in that roster is full ... of mediocre players. There are like 14 or 15 forwards, and only 4 of them are genuine top-sixers who should be entitled to spots (Monahan, Dach, Caulfield, Suzuki). So Roy will go down to the AHL because he doesn't go through waivers and he can develop in the AHL just fine as he will be facing a steep jump from the Q.
Point #2.5), Roy dominated the Q, a relatively weak junior league, so he can learn a lot by adapting to the AHL, it's a big jump. In contrast, when Reinbacher will have completed 2 years of dominating the Swiss league, there will not be an argument that he can learn a lot in the AHL.
Third, it might be a really nice environment in Laval. He's coming in with a lot of other young players like Struble, Mailloux, Mesar, etc, so they might have a lot of fun and grow together and be part of a future Habs core.