They aren’t wrong. You’re keeping the mass constant, they’re keeping the volume constant.

I think the confusion might come from their phrasing: “the same volume of ice as water,” which could mean “the same volume of ice as the volume of water” (which is what they meant), but could also be interpreted as “the same volume of ice in the form of water.” The latter interpretation doesn’t fit the rest of their sentence though, so we can safely assume they meant the former.