I don't think it's starting to cut from the far end of the part. It looks like it is placed on the surface and digs in to start.
The block then could be however long they need.
It also looks like the fin length is shorter than the cut distance, material compaction making it shorter?
The lifting of the fin to straight seems like a good place to crack off a fin and scrap the part.

Obviously it works, wonder what material it is. Gotta be ductile and somewhat soft to cut like that.