He did discover Mahrez. Leicester sent him to watch somebody else but Mahrez stood out and that's who Walsh recommended.
That's splitting hairs, anyhow. Koeman has no expertise at evaluating and signing players (see: Martina, Cuco). Every club at which he's succeeded (the three big Dutch clubs, Southampton) has had a sporting director of some type. We shouldn't be happy with Everton's recruitment over the past three windows because it hasn't been good enough but Koeman is not the answer.
The manager's primary job is to prepare the players for matches and help them improve on their weaknesses. He should be spending all of his time doing that. Recruitment is a time intensive job that distracts from a manager's true responsibilities. Or, to put it another way, do you really think Koeman has enough time for coaching/preparing his team, finding suitable transfer targets, playing golf, and going on vacation?
The do-it-all manager model is outdated, as evidenced by there being zero top teams in world football that let their manager control everything. It is 100% right for us to have a director of football. Whether Walsh is the best person for that job is another question and hard to analyze since none of us are sure how much control he has.