Over the years, I’ve dealt with code which had tests (unit, functional or both). Some of it was even test driven. But it was extremely difficult to understand and maintain the code. The code-base exhibited the same problems as Legacy code.
What does this mean? IMHO, it means we need to broaden our definition of Legacy Code.
Legacy code is code that developers fear facing. Legacy code does not communicate its intent and has a very convoluted design. It is code with high viscosity which encourages sloppy job by the developers and makes it extremely difficult for them to do the right thing. Abundance of Code Smells, lack of Tests, long feedback cycles, unpredictability, etc : all of these are contributing factors.