The 400 Mbps data rate is the per-channel switching limit; at that speed the receiver's differential input threshold and 50 mV hysteresis define the noise margin. If your cable run exceeds a few meters or passes near a VFD, the hysteresis helps prevent the output from chattering on common-mode noise coupled onto the LVDS pair.
The supplier device package is 16-TSSOP, matching the package / case field.