I've heard the same, but I'm kind of skeptical -- I haven't actually researched it, but the factoid usually seems to come up when someone's trying to push a narrative, which tends to be a warning sign.

In particular, if mortgages and automotive debt are being counted, then most adults before retirement age might look pretty indebted on paper without that necessarily destroying their medium-term ability to stay solvent. On the other hand, standard financial advice seems to be to maintain six to twelve months' worth of savings, which suggests to me that most people don't.