Microsoft didn't say Windows 10 S was unhackable. They said that no known version of Ransomware can successfully attack a Windows 10 S machine, which is completely true. The ZDNet article described a manual, human driven process to install potential malware.

In other words, nothing that is currently in the wild will infect a 10 S machine automatically.

That's not saying much though. Since new malware could be written to specifically target 10 S, but it's doubtful anyone will bother since 10 S will likely have a pretty small percentage of users.