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.