Had a quick search for other documents about the biofilm class action, and came across a few later ones.
These mention biofilm deposits leading to crevice corrosion, and also that the use of hypochloride bleach accelerates this corrosion.
As I understand it crevice corrosion can occur where there's a small void like pitting, or a junction between surfaces (eg spider and the drum) and oxygen get excluded, by biofilm in this case, the water becomes increasingly acidic, chlorine ions make this worse and oxidisation of the aluminium uses up all the oxygen, so that the aluminium can no longer form a passivated aluminium oxide coating.
Something similar occurs if you were to use stainless steel screws below the water line in the hull of a wooden boat used in salt water and seal them with stopper.
There was also this about the causes of the increasing occurrence of biofilms and smells.
" The March 1, 2006 Whirlpool Document discusses why biofilm and corrosion
were becoming an issue at that time. The document attributes it to changes in
washing habits (fewer high temperature programs, increased use of liquid
detergent with reduced corrosion inhibitors, short cycle time has priority leading
to full load being washed on express cycle with insufficient rinse, market
requiring big load capacity), wash programs using less water at lower
temperatures leading to poor cleaning of the inside of machine and the fact that
the Washing Machines are basically a European design, not necessarily suited
to US washing habits (low water temperatures, HE detergent not always used
and widespread use of bleach in quite high quantities). The discussion also
identified “lack of specifications and poorly understood design concepts”: "
https://www.clg.org/pdf/4/3/7/1/Amended-Application-to-Institute-Proceedings.pdf
https://www.clg.org/pdf/7/7/9/New-Motion-to-Authorize.pdf