It’s becoming too mainstream is what they mean. Any other not immediately obvious parallel to Windows would have warranted an explanation from the get go.
Not too mainstream, and i still use Linux in a ton of places. Mostly freebsd is simpler and more consistent; having the whole operating system in a single place means that the documentation is better and I don't have to worry if some setting is in /etc or in some database somewhere or in one config language vs another.
I still really like Linux, but I think it's healthy to have multiple viable open source operating systems.
Linux on servers has been very mainstream for last like 20 years.
Big distros like Ubuntu become ever more complicated though, they run a ton of stuff by default, like Windows. OpenBSD is, first of all, very small in comparison. Then, it's obscure enough that most online attackers do not target it.
There might be a correlation with mainstreaming, but that's not the root cause. I'm happy to use a mainstream OS if it is awesome.
A full honest answer is too long for a HN comment, could write a long article about it but won't do that.
I need an OS which is fully comprehensible and fully configurable to my exact taste and fully diagnosable at all levels. So basically the exact opposite of windows which is a massive blob of obscurity. Mac OS has been great for a couple decades, but the enshittification accelerates. As a tiny example, dtruss doesn't even work on newer Mac OS. That kind of nonsense is unacceptable.
An OS is supposed to be a platform & toolkit to enable me to use and diagnose the computer any way I need. Linux is still that, sure, but enshittification is creeping in. Wayland/systemd/gnome/etc is all movement in the wrong direction.
I still really like Linux, but I think it's healthy to have multiple viable open source operating systems.