I'd argue code I don't understand is indeed tech debt. not sure what you were implying with your statement, but I sounded as if you disagree with the statement. undocumented code that no one knows is a serious tech debt issue. time should be taken to read it, understand it, and document it.
Agreed that code that nobody in the team understands is tech debt; code that only a few people in the team understand is a liability, but not directly debt. Whether you understand the code yourself shouldn't immediately cause it to be labeled as "debt", unless you're the I in team (which can easily happen for personal projects or badly-managed organizations).
