@Jack was hacked. Wired has a story how it happened. TL;DR: Sim Swap is an attack that allows intercepting the 2nd factor for authentication, when using SMS to a cellphone. Source: How Twitter CEO Jack Dorsey’s Account Was Hacked
Stribika gives a bit of advice on how to make a ssh server secure. The advise is focusing on strong cryptography and authentication, along with specific configuration examples for openssh.