The Dutch Wiki covers that: he was head of the academy at Twente from 2002-2006, and additionally an assistant coach for the first team from 2006-2009. He was assistant to Rutte at the end, and then followed him to PSV, where Ten Has was assistant coach from 2009-2012. After that, you get Go Ahead Eagles, Bayern II, Utrecht, Ajax, United.
https://nl.m.wikipedia.org/wiki/Erik_ten_Hag
So yeah, definitely not the kind of guy that blundered straight into a high-profile coaching job right after his playing career!
Those early days might also explain why he appears to have some strongly held views on the links between the first team and the academy and reserves: he's been part of those sides of clubs for years as well, he knows what he's talking about.