The test was flown on a DC-8-72. This model aircraft is certified to have its inboard thrust reversers deployed during flight, which means that the test was either within standard operational guidelines or not too far outside them (
it's possible it's not supposed to have reversers deployed at Mach 0.8, but it's allowed to have them deployed at speeds over 190 knots per @UnrecognizedFallingObject's comment, thrust reversers are allowed up to Mmo). In addition, both inboard thrust reversers were deployed; the TV show is wrong when they claim that there were 3 normal engines and one reversed one. As the test found, there was some buffeting, but there wasn't the need for too much lateral control input, because of the symmetry (there was a bit of asymmetry when deploying reverse thrust, but not once both were deployed). Also, crucially, the flight crew was prepared for this, and they had the outboard engines fully spooled up (so while there was high sink rate at high speed, they had the power to come out of the dive safely). The test protocol had them maintaining airspeed, idling the reverse thrust engines before the end of the reverse thrust run, and then having engines at full thrust already to pull up.
In short, the reason it wasn't a suicide flight was that the flight crew was prepared, the plane could take some degree of reverse thrust in flight, the reverse thrust was symmetric, the outboard engines were both spooled up and pitch was adjusted to maintain airspeed, and in general it was a controlled test environment. The TV show implies it was to see if it was possible to recover. That was not the purpose of the test; they set up the test so they knew they could recover, and the purpose was to see the effect on airflow over the wing in as controlled an environment as possible.
Source: NASA report on the test flight