Reading the quote in the OP....
"temporary buydown"? Sounds like an ARM....wonder why it isn't just called that?
I don't know about the "subsidy" thing either....I don't know why a seller would be willing to be making up the difference in a buyer's mortgage payment for a couple of years "until the buyer can handle the higher payments". Certainly wouldn't happen with an FHA or VA loan....if the buyer can't qualify for the payments, they don't get the loan. The requirements have always been pretty strict, and those loans are very specific as to money coming anywhere other than the buyer's pockets.
Buying down interest points up front, can be quite expensive. 1 point = 1% of the amount of the loan. If the loan is $100,000, then 1 point =$1000.
Refinancing a loan to lower interest rate. The amount of the reduction in payment, needs to be compared with how much it costs to refinance the loan - the lenders don't do it for free....no matter what they might tell you to get you to do it. That cost then needs to be compared with how long one expects to be in the house. Usually the interest rate has to be at least 2.5-3% lower to make the costs worth it, as the costs usually are 3-5% of the amount of the loan - as if the house was being purchased anew. Those costs are either up front or attached to the amount of the loan.....increasing the original amount being refinanced by those costs. Most often, it's attached. That also needs to be taken into consideration as to deciding if it's worth it. Pretty risky in today's market, unless there is a lot of equity in the house prior to refinancing.
It doesn't matter how low the interest rate is, if it increases the amount of the mortgage to the extent that there is no equity or that the house can no longer be sold at a price high enough to recover the costs of the mortgage. It is not unusual for a lender to add a penalty either, if the house is sold within a certain time frame. I had refinanced my house with an ARM and sold it 45 DAYS before the minimum time was up and it cost me $7200 penalty. I was pissed to say the least, but there was nothing I could do about it.
A mortgage lender can explain it much better than I can.
It's the same as with most other things though....if it sounds to good to be true....it probably isn't exactly as it appears.