The issue was never whether ash can cause damage to aircraft and engines, of course it can, the experiences of BA in Indonesia and KLM in Alaska are well enough documented.
The issue was with the critical density within the cloud and at its margins, how it was moving and how it was dissipating; I haven't read the report, but does it really make a convincing case that a sufficiently high risk necessitated the closure of airspace as far as the Adriatic coast within the first couple of days of the event? If so, how was it that this risk had suddenly disappeared by day 4, when almost all the affected airspace was reopened, leaving the 'danger area' where it had been all the time, a relatively limited region, at a relatively limited altitude, over the sea between Iceland, Scotland and Norway.