* The dictionary gives me "debt (n): a sum of money that is owed or due". To define (or even describe) money in general as debt with this definition is obviously circular.
* If you simply define "debt" as "money" then to define "money" as "debt" is obviously even more directly circular.
The point being made isn't really wrong or unhelpful. As you say, a person's bank account is a liability of the bank, so to say "I have money in a bank acccount" really means "the bank has an obligation to me", and a lot of "money" is like this. Not all, though: cash and bank reserves can't (in the modern world) accurately be described as debt in the sense of "somebody owing somebody money" (and it's debatable whether even an individual bank account is fully describable that way).
There is no easy, trivial definition of what money is. Probably the most general view is that it represents some sort of obligation or claim the person who "has the money" holds over others. That's a valid insight, but it's too general to be a definition as it's also satisfied by many things we don't generally think of as money, eg. train tickets or stocks.
The "money is debt" notion is, I think, an attempt to express this useful-but-too-general "claim or obligation" idea, but given people usually think of "debt" in a much more specific sense as "money owed" I don't think it's terribly helpful for people who are struggling with what money "is". At best, I think it would have to come with a lot more explanation of exactly what is meant by "debt" in this context, and would need caveats to avoid being overly broad.