Might be true, but as someone who worked on a few legacy projects, the opposite, i.e. "You're not a senior engineer until you've worked on a greenfield project" is also true. There are skills that you won't pickup by working on old piles of code. Knowing what technology to pick, how to design your system for high-availability and how to avoid needless complexity are just as important to a well-rounded senior.
Perhaps in the end you are not a senior engineer until you've done a bit of everything.
Personally reading this thread I just realized that devs encompass so many areas. Requirement for senior engineers working on practical formal methods, real-time systems, Linux-kernel hacking, or those writing CUDA kernels all day are all gonna be different. System design is only for a broad kind, but not for all kinds, of senior engineers.
I agree with you. But, I have found the middle out effect in online discourse means we get articles (such as this one) polarising viewpoints for attention. Sadly, it works.
Perhaps in the end you are not a senior engineer until you've done a bit of everything.