Mac was on my list of highly secured OSes when it comes to viruses and other forms of attack. But it's obvious that no system is completely safe and immune; those hackers just have to make it their primary goal to develop something that they will use to penetrate any form of OS they want.
The source or how the malware spreads are not listed on the tweet. Generally, without a doubt this kind of malware are coming from unlegitimate or unofficial sources, regardless of any OS. So it is essential to use non-cracked/pirated software and make sure to download it from legitimate sources. They also might use either or a combination of social engineering and phishing.
A protective measure from any antivirus or OS securities can be bypassed, as initially happened with the malware, but the user itself is the last defence.