Nice! In today's environment, 7% guaranteed is amazing!
Wonder how sustainable it'll be (in regards to pension promises and such).
A TDA is an annuity. Based on that fact, I would assume that the annuity was sold by an insurance company to the NY teachers' organisation. The insurance company is on the hook for the 7% return guarantee, and in exchange they were given capital to invest (to try to earn a >7% investment return) plus some fee for writing the guarantee. At the time that they write the guarantee, the insurance company builds an investment portfolio of assets and derivatives that they believe minimizes the risk of shortfall.
Presumably the insurance company wrote the contract at a time when 7% return seemed reasonable to them. Now it certainly seems quite high, so you'd hope (for their sake only) that the insurance company has made some big profit on the investments and hedges they made. Of course, they are legally obligated to pay the 7% regardless, for the duration of the contract they agreed to.
When the contract expires, the NY teachers' org will need to buy a new annuity (or other "stable value" product) from an insurance company. In current market conditions this would most assuredly be at a <7% rate.