Don't believe him - capillary action happens with polystyrene balls, fluff get wet, so does rockwool and glassfibrous stuff.What kind was it?
They say you should never do a wall that faces driving rain.
Lots of my old neighbours had it done back in the 80s. Serious damp problems and galv wall ties having to be replaced.
Apparently "modern" stuff is better?
My old house is semi derelict. Long story but finally in a position to have it renovated. I can get a grant for a heat pump and solar panels which will save me a fortune. Surveyor drill into cavity. 3% moisture content. Never been insulated. He was astonished.
The whole pine end is very exposed to driving rain but he "assured" me modern stuff is fine.
No damp at all in an unoccupied house empty for 10 years with no heat is pretty amazing.
If your wall is south facing and exposed - don't do it!