I'm a qualified car driver. But I wouldn't class myself as an expert on the composition of asphalt on tyre grip capabilities based on varying levels of remaining tread depth vs road surface wear etc. Nor on the relative effeciency of halogen headlights vs newer xenon beam types in varying levels of darkness / fog / rain.
So unless any pilot has flown through a number of varying concentration levels of volcanic ash and directly assessed the effect (supported by experts from the engine manufacturers) I would suggest the "based on expert knowledge" is stretching it a bit
I do like
Jumbodriver's acid-in-the-pool analogy though ^
One more thought about the acid/pool analogy.
One cup of volcanic ash blown over an airplane at 500 miles per hour will sandblast the paint off and melt on delicate engine components.