I don't believe IQ is measurable as much as it is noticeable. It is not easy to detect watching highlight clips. You need to analyze the players entire shift in a sample size of as many games as possible.
Hanging in the weeds and picking off passes is a form of high hockey IQ, which is the reason why I don't think any one that is moderately knowledgeable about hockey believes that Byfield has a low IQ. That form of IQ is what I would label reactionary Hockey IQ. Being able to recognize and take advantage of opportunities as they present themselves. That combined with Byfields strong positioning(anticipatory) and excellent vision(both depending on scenario) make his overall IQ on the higher end of the spectrum, which combined with his elite technical skills easily translates to top 10 talent.
The critique on Byfield is not that he has a poor IQ, but that in the other form of IQ Byfield is not off the charts to the degree of... well... a "franchise" player. The form of IQ not off the charts is what I would label as anticipatory IQ. This does pertain to the defensive side, but typically I give it more weight towards offense. Off the puck is an important component and that is truthfully an area Byfield's IQ is very high. Players with the highest anticipatory IQ operate on a level where their brain is multiple steps ahead. It is really just the extent a player understands/processes the game and is able to predict then create outcomes. They rarely if ever make the wrong read or turn the puck over. Most importantly play seems to gravitate around them. Not only are they good, but people on their line benefit just from being around them. Crosby and Gretzky have the highest anticipatory IQ ever. Seriously, it is very hard to describe but just watch and analyze sidney crosby in any game he's played since 07-08. Not his highlights, him in a full game. The multiple steps ahead thing should be evident. Chess vs. Checkers. Crosby's game is ultimately the epitome of high hockey IQ in both forms.
In this draft Rossi and Perfetti have the best anticipatory IQ. They are not ranked higher because Rossi is short and Perfetti needs to refine technical skills(skating). Also I put franchise in quotes because I feel like that specifically connotates a generational talent.