Translation:They wear white underwear.
Actually, no. パンツ is just like "pants" in English in that some people use it for underpants and some use it for trousers.
If you do an image search for it, you'll mostly find men's underpants, but also women's knickers/panties, women's slacks and occasionally things like knee-length shorts or women's culottes.
UK English 'pants' should be accepted. 'He' is incorrect, though.
Also, as we're using the verb for wearing lower-body clothing, 'underwear' is a sloppy translation.
Technically speaking, 'underwear' is a general term for undergarments in American English. It also includes socks, undershirts, garters, etc. British English 'pants' is American English 'underpants'.