Do high-end brands even produce 300 kilotons of clothing? Assuming, very generously, that a piece of clothing, with packaging and all, weights 1 kg, it would be 300M pieces of clothing; that could be an entire production run of something very ubiquitous (say, Levi's 501), but definitely not high-end.

I think that tonnage is for all textiles, not just high-end clothing.

https://www.eea.europa.eu/en/analysis/publications/the-destr... says "Based on available studies, an estimated 4-9% of all textile products put on the market in Europe are destroyed before use, amounting to between 264,000 and 594,000 tonnes of textiles destroyed each year."