this exposed rebar had virtually no concrete cover for protection from moisture
Horizontal cracks in concrete are almost always due to inadequate protection from moisture. Remember that concrete is porous. Moisture will penetrate into the concrete. For that reason, the approved standard is to place and secure the rebar so that there is at least 3-inches between the rebar and the surface of the concrete.
Note the exposed rebar appears to have originally been placed barely beneath the concrete. There was probably no more than a 1/16th inch of concrete covering the steel rebar.
If you look closely, it appears that the rebar is in remarkable shape considering that is clearly to moisture penetration.
At some point, this rebar should be repaired. Steel rebar that is damaged by moisture can swell will swell in volume five times the original volume. This can cause severe damage to the rebar.