It is guaranteed - by Tether. But the effectiveness of this guarantee is what's questioned, and the only way to fully validate this guarantee is to wait until the USD have left any accounts controlled by Tether and arrived in your bank account. You are inside the danger zone and outside of the profit zone until this process has completed. And this process is not instant at all.

[edit] Indeed, as dgellow commented, Tether actually does not strictly guarantee the redemption of USDT to USD. They use sleazy legalese in their Terms of Service to reserve the right to "delay" a redemption in case of illiquidity, without specifying how long that delay may last, so effectively they can simply "delay" your redemption for an infinite time.

It’s not guaranteed at all. Tether made it abundantly clear they give no guarantees they will redeem your USDT. It also isn’t instant.

This is public knowledge and has been well documented for years. Tether is likely to be the least trustworthy actor in the entire industry.