Yes, Absolutely agree mate, soap flakes are ideal in normal circumstances, but the op's issue is an item that was left wet for some time in a poly bag, this would let bacteria grow, resulting in the stink. to deal with that in a washing machine by itself would take biological detergent on a 40 or 60 degree wash. And I would not recommend
anyone putting their gear through that.
Washing and drying, then freezing the item for a couple of days would kill any bacteria, it can then be rewashed at 30 degrees with soap flakes to remove any residue. Good as new
Freezing is also a great way way to de-pongify boots.