Either Sing Out got it slightly wrong in 1962, or maybe the writing credits were 'harmonized' later on:

[1962:] Peggy Seeger wrote this song after reading about the terrible mine disaster in Spring Hill, Nova Scotia, in the latter part of 1958. This was the world-famous tragedy in which a number of trapped miners were miraculously rescued after eight days of entombment. (Reprint Sing Out 4, 202)

[1970:] The Springhill mining disaster of 1958 was the first to receive world-wide television coverage. Ewan and Peggy saw it during a visit to Paris and this song - a joint composition - was the result. (Notes 'The World of Ewan MacColl and Peggy Seeger')

On www.peggyseeger.com the song is listed as Peggy's, not as Ewan's.