They are not demanding ransom. Ransom is (per Merriam Webster): "a consideration paid or demanded for the release of someone or something from captivity".

They copied the data, and they want money otherwise they will release it. It's ordinary blackmail.

Perhaps you could say they are ransoming the exclusive ownership of the data. But yes, 'blackmail' seems like a better fit.

The very first sentence of that link would include this under "ransomware"

> Ransomware is a type of malware from cryptovirology that threatens to publish the victim's data or perpetually block access to it unless a ransom is paid.

(added emphasis)

That's a single sentence pulled from a very long definition, though.

Here's the third sentence from that very same paragraph:

>It encrypts the victim's files, making them inaccessible, and demands a ransom payment to decrypt them.

Not everything can be explained in a single sentence.