Sure, all teams have some data info and scouts/advisors who weigh in. I posted about the Leafs data on this site about 20 years ago because I got to see some of it and how it was collected. But someone has to take all that in and make the final decision in the heat of the trade deadline. If the GM knows little about all the players involved and has to rely totally on data, opinions and scouting reports - from staff/advisors he doesn't know well, etc, then the scouts, AGM and staff probably should be making the decision - or knowledgeable posters on the internet might be better

. A guy out of the league for 17 years isn't going to know his adversaries very well either. All of that is hard enough to assimilate for a GM who has bee around for the last 17 years
The notion of Sundin being a GM out the gate was crazy talk.
It certainly helps to be familiar; I think it also introduces additional bias into the proceedings if the GM/HOHO already has too much familiarity, so to speak.
No GM knows every player/prospect; good data-driven GMs will set up processes and procedures to
a) surround him or herself with people whose instincts and skillsets complement and enhance the team
b) gather as much data as possible (hard stats and soft data like character, response under stress, competitiveness, etc.)
c) calibrate the tools for that sort of data lookup to isolate the signal from the noise
and then when decision times come around, use the data and those trusted people closer to the situation (as well as outside observers with less personal investment) to reach an informed decision. It's a job that requires knowing your own people more than knowing what pieces are out there, which is the job of the supporting managers/operators.
Like, the President of an auto manufacturer really doesn't need to know the nitty-gritty details of how parts are stamped/welded or the specific alloys. But that person should understand the identity of the brand and what it aims to accomplish, and set the VPs and Directors on the path to achieve those aims.
To that end, Sundin has lived it, and has some understanding of what he would've wanted from the org to help them succeed. He loves the Leaf, but has no particular attachment to the current way things are done, which is kind of perfect for a reset situation. How many years he's been gone is not super material to the matter as long as he is opened minded about how the game has evolved in systems. For that, he has a front row seat to the development of Swedish players to eventual NHLers, and the Leafs have oodles of data about successful teams and systems.
I also think Auston Matthews, and to a lesser degree William Nylander, have significant pull here as well, because whatever plans come to fruit need to align with their goal of winning here sooner rather than later.