As evidence of how Gretzky saw their relationship: Marner and Matthews reminds him of it.
I don't think anyone would argue that Kurri (or Messier) was the better or more talented player, your argument has been that Gretzky was the primary (as opposed to secondary) reason behind the success of these players and what they did on their own does not at all fit that picture.
Gretzky undoubtedly was the most talented player in the game, but he was also only a player in the game, not the otherworldly demigod you try to insist he was. Besides his immense talent and its implications on team hierarchy he was a teammate like any other, a young one at that.
Many of the greatest mentors were not "singular talents", besides Matti Hagman take Brad McCrimmon from Gretzky's WJC team, he never became more than a solid defenseman in the NHL, but as a veteran he strongly influenced several that went further than that.
Statistics are quite frankly uninteresting, hardly anyone cares whether Messier peaked at 107 or 129 points. It's what he did in '84, '90, '94 that is remembered.
The use of secondary (first) and primary (second) effect is applied as follows: Gretzky's assists are a secondary effect. His primary effect was his day-to-day example as evidenced by Messier. When I made the comment about Gretzky's greatness being so exceptional that his secondary effect (i.e. assists) contributed to the success of players around him, it was isolated in the sense of production.
And the caveat is always going to be: Kurri has to have the ability to read Gretzky and then convert the chances provided. We are in complete agreement that Messier and Kurri were great players and were likely to become great players without Gretzky.
All-time great is a separate category.
When I note Gretzky's primary effect, it's with respect to overall development. But it's not the starting point despite the use of the word primary. Obviously - to repeat - the players were great in their own right.
For lack of a better example, Bruce Wayne in Batman Begins is a pretty resourceful, pretty tough individual before he hikes his way to The League of Shadows. On his way to becoming Batman, which takes trial and error in Gotham, he has to navigate the gauntlet of the League's entailments, and does so, over and over as if to identify himself with distinction from the League, certainly in ethos; But...Theatricality, deception, fighting, even his gloves have their beginnings because of one source: Ras Al Ghul. By the end, Batman's greatness is his own. But Batman will never be Batman had he not come under the wings of Ras Al Ghul; Al Ghul had a primary effect on Bruce Wayne actualizing his potential that without the knowledge and example of the League of Shadows never manifests in the future as it unfolds.
All things Gotham: Mark Messier emerges as the better leader in my estimation. He is a better captain than Gretzky, as only hindsight would provide. But he is not a better leader in spite of Gretzky. He becomes the best better player version he could and in turn a better leader because of Gretzky, as his testimony recounts.
The reason Mark Messier and Jari Kurri are such great players is solely down to their ability and industry meeting opportunity. They are agents of their own destiny. Gretzky's primary effect on their ability and industry falls more into the opportunity category.
They were going to be at the very least, very good players. With Gretzky, they're all-time greats. Without Gretzky, we don't know.