Even leaving out a lot of expensive business/formal clothing--which is less common in most circles these days--specialty outdoor activities clothing and footwear adds up in a hurry, especially if you do stuff in cold/snowy winters. It wouldn't surprise me if my winter outdoor gear added up to that by itself. I admittedly have way too much old clothing I rarely wear (and even more that I really wouldn't miss) but I'm sure my inventory is still way more than that.
Funny, I looked at his $3,907.65 total estimate and though that it's quite low for an adult.
Consider though that OP used retail prices, not thrift shop.