At the time KLM made the "no adverse" announcement on Sunday they had not yet taken the engines apart for inspection. So all they know is that the engines didn't fail during that flight, not whether they'd survive even another 100 hours without failure.

NATO has taken apart a couple of engines from fighters that did have damage from this ash cloud.

The ash cloud does not extend uniformly from ground-level upwards: vertical wind sheer means many altitudes are ash-free, yet a few thousand feet above or below have ash.