No, they likely are not. According to their Sustainability page, they aim to democratize sustainable fashion by producing on demand, reducing waste, and using recycled materials. They allegedly maintain an accountable supply chain, ensuring equality, health, safety, and fair wages. They are transitioning to certified organic cotton by 2023 and repurposing plastic bottles and textiles. Since they don’t substantiate any of these sustainability claims, I am inclined to think they are green-washing.
Check out this article What Is Fast Fashion, Anyway? by the good trade to learn more about fast fashion, sustainability, and how to spot it from a mile away!