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.

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.