Not necessarily. If the rewrite is only addressing issues that would have been prevented with tests, then sure, this is clearly an argument for TDD. However, if the rewrite is going beyond what could have been provided by tests, then having a large testing system could actually make the rewrites harder, which means it's an argument against TDD.
It's hard to say which is the case without knowing the details, and I'd certainly err on the side of saying a yearly rewrite is not a good sign. But if a company is undergoing rapid growth, it's not that unusual for certain systems to be rewritten frequently as fundamental new insights are gathered about how to tackle problems that are hard to scale. And if the system isn't that large to begin with, periodic rewrites could be easier than writing a single version that's supposed to last 10+ years as business requirements dramatically change and expand.