Hi Kazem,
Welcome to the TI E2E Forums!
The differential input impedance is impedance while the ADS1258 is sampling the input. The switch time delay will postpone this sampling; therefore the input impedance will be larger, but only momentarily until sampling begins again.
This delay helps in the case where a large filtering capacitance between the MUXOUT pins and ADCIN pins must settle after switching multiplexer channels.
We still recommend adding buffers to the inputs of the ADS1258 for best performance.
Best Regards,
Chris