I still don't know how that hacker was able to breach my account. I tried to recap what I did these past days and weeks. As an internet guy for a long, basically, I know the basic and advanced safety measures for protecting my personal account. Although there's no 100% guaranteed protection, I'm really sure I didn't do some crappy moves that lead to this. My Bitcointalk account was only accessed here in my IP's home and on my mobile data (single network) when away.
Perhaps some cybercriminals were able to extract your Bitcointalk session cookies and sell them on a darknet marketplace while you were away (this can be mitigated by logging out and back in again).
Or maybe a low-tech explanation is that somebody got access to a device you were logged in to Bitcointalk with, or they got your username/password, and then they started to make the malware posts.