Calculating thermocouple voltage error due to ADC anti-aliasing filter resistors
The input offset current appears to be an offset current as you suspect.

Note that the datasheet has typical values.

It does indeed look like there will be an input referred error term of \$V_{err} = I_{diff} \cdot R_{source(total)}\$

The typical performance curves tell an interesting story:



I would also note that there is a total input referred error term:



Whether that input referred error graph includes the input offset current is not clear; I would assume for the purpose of managing the error budget (from the legendary Bob Pease) that it is not.

The input currents vary with input voltage in one specific scenario, apparently:



As this statement is specific to this single line, I would assume that in any other mode the input currents (both absolute and offset) do not vary with input voltage.

[Update]

In response to the comment, it is extremely challenging to make a thermocouple circuit accurate to within 1 Kelvin (some might go further). 

It is possible to make the differential measurement (i.e. the temperature change) accurate to this level with some initial span measurements (setting the thermocouple to known temperatures to calibrate out the INL, DNL and gain errors although you would probably need to maintain the ADC temperature constant), but an absolute accuracy at this level may simply be too much to expect.

The input voltage error term seems to be a rolled-up total of the input voltage error of the amplifier(s) and the ADC errors. 

I do not know what type of thermocouple you intend to use (they each have their own transfer function), but any calibration would be best done in software as any offset circuitry will introduce its own errors.