The floss itself is made from PLA, a 100% bio based-plastic, derived from plant-based materials, in this case, corn starch. This makes it 100% plant-based, but it has been processed to PLA which can fully decompose to natural elements under the right conditions.
This floss is vegan, but not fully compostable in your backyard compost.
You may not be able to compost this floss in your garden but is it biodegradable under certain temperatures and enzymes, this happens mostly in an industrial compost where all products made from bioplastics are recycled.