From the linked article: «Namely, in iOS 10.1 and 102 (sic!), metadata for APFS was test written and the superblock header was created but not actually written

From the linked article: «Namely, in iOS 10.1 and 102 (sic!), metadata for APFS was test written and the superblock header was created but not actually written out. The file data remained untouched for safety and crash protection and the user remained in HFS+.»