I can't vouch for this one, but I did use one of their other suggested methods (involved washing) for a particularly smelly down jacket, and it worked a treat.
The suggestion (without washing/getting it wet) is to use baking powder. Sprinkle it over the smelly area and let it sit for 30 mins. After that, shake the powder off/vacuum and let it hang outside to air it out for a while. I'll be trying this method next time, but let me know how you go if you give it a crack.
The suggestion I used was to add a cup of vinegar to the rinse cycle of the wash. I ended up pausing the wash, bc I was having issues with the machine, so I let it soak in the vinegar/water mix for half an hour or so, before continuing with the rinse cycle. I was pleasantly surprised at the result - didn't think anything was going to work to be honest, and it came out perfect. I'd previously tried just a regular wash with down-specific detergent but it failed to remove the smell.
This was my reference (just need to scroll down a bit):
https://nomadhiker.com/how-to-get-smell ... er-jacket/I only had access to a top-loader and didn't see the warnings against using a top-loader (on both website and washing detergent bottle) until too late. Jacket was fine. Tumble dried with the balls, no/very low heat - took ages to de-clump and dry, but got there eventually.