How does isolation prevent input signals to float/saturate for differential ended inputs?
Those input channel devices are not op-amps, but isolated analog signal conditioning modules.  Internally I suspect they contain op-amps, but that is transparent to the user.  All you see are connections for power, input, and output, and a set of specs that characterize performance.  Bias resistors are internal, manifesting themselves externally as part of the input resistance spec.

I believe the one who told you it is "Truly Isolated" meant it is galvanically isolated.  The inputs are floating with respect to ground and the power supply common.  A block diagram of the 8B voltage input module is shown below, obtained from here.