It’s open source, which in this case means a lot of people are working on it, and it is used on internet-facing computers, which means it can’t hide behind a residential firewall so has to be secure against every attack imaginable.
So yeah, the fact that is called merely “secure” is indeed lol, it should have been called “ultra mega super max secure”.
This article is just one of several articles that cite vulnerabilities that GNU/Linux has. Open source is something I advocate but I am aware that the mere fact that a software is open source does not guarantee that it is secure. Many eyes checking the code did not prevent the xz Utils backdoor from being included in several distributions. The monolithic kernel is also problematic every day, more and more lines of code are added, further increasing the attack surface, among others. Just to name a few