IANA Cryptographer but as far as I know, the initialization vector (IV) of a cryptographic protocol is usually public and an integral part of the protocol specification. Both parties must agree on the value of the IV so they can understand each other. If the IV were secret, some kind of handshake (similar to key agreement) should take place before the connection is initiated.

I don't know how using a block cipher in cipher block chaining (CBC) would be different here. In the article, OP suggests that it may be related to short messages. Indeed, if the plaintext is less than or equal to one block, and you re-use the same key (which you should never do) it's possible to do replay attacks.

If anyone has deeper knowledge of cryptography and block cipher modes in particular, please explain to us how CBC is vulnerable if the IV is known by the attacker? Why are short messages more vulnerable and to which attack modes?

As I said, I'm not a cryptographer, only a uni student with an exam on crypto tomorrow. To me, it seems like the article had a linkbait title and it didn't describe a very big threat anyway (and it was only superficially related to AES). But if crypto class has taught me anything, it's that intuition often fails. So don't take my word for it, there may be a real threat behind this.