Net worth literally means assets minus liabilities.
If you have a mortgage that isn’t at least 50% paid off you probably have a negative net worth, while a renter without other debts will have a positive worth.
The accumulation of equity compared to the complete loss of value to renting is a different discussion, since they mentioned they just purchased and are only 18 months into their payments.
This discrepancy, btw, is why people accuse Trump of being a fake billionaire, because his liabilities supposedly exceed his assets, thus his begging for cash from the gullibles.