Phishing actually is a core branch of hacking—specifically under Social Engineering. It's not really like walking through an unlocked door; it's more like a con artist dressing up as a locksmith and convincing the homeowner to hand over the keys.

Hacking applies to the entire attack surface, which includes the human element, further more there are whole phishing campaigns that are heavily automated and often deliver stealer malware, making them a full cyber attack.

This wasn’t a technical compromise of Signal itself, but phishing/social engineering is still a form of hacking.