They're saying your interest payments may feel like much today, but because the currency devalues over time/inflation, it remaining constant will end up being cheaper in the long run.

Today a 30 year fixed at 6.48% for 400k with 80k down payment is $2,400 / month with insurance and property taxes, but let's just say 2,500. That currently will rent a nice 3 bedroom apartment in mid sized metropolitan areas. In 30 years the mortgage might be 3,000. What would a 3 bedroom apartment be renting for? 30 years ago it was 1,200 ish. So 30 years from now, 5,000 for that apartment?

Mortgages make sense for the long term, not just equity (house value goes up) but stability in your monthly housing budget.

That mortgage will end up costing 862k, not 400k. But let's be clear, 6.5% interest is horrendous, you can get a lower rate sometime in the next decade. This plan makes even more sense if the money you save as the years roll by you invest in retirement instead of getting a boat, motorcycle, or avocado toast.