By using materials whose flame resistance is structural rather than applied. Avocado uses certified organic wool in every non-vegan mattress — wool's keratin structure makes it inherently difficult to ignite without any chemical treatment. The Vegan Mattress uses expandable graphite, and the Vegan Latex Mattress uses hydrated silica for the same purpose.
All three approaches meet federal flammability standards under 16 CFR Part 1633, verified under GOTS finished-product certification (license CU863637), MADE SAFE®, EWG Verified®, and GREENGUARD Gold.
For the full story, see Why Avocado Uses Organic Wool Instead of Chemical Flame Retardants.