Voltage reference output impedance calculation
Short story: the accuracy of the AD7190 is not 24 bits; it's less than 18 bits.
Let's put this problem into context. The AD7190 (B grade version) has these errors: -


INL of 5 ppm of full scale maximum
Offset error of 75 μV typically
Gain error of ±0.005 % (50 ppm equivalent)

The ADC has a conversion resolution of 24 bits and that is far lower than the accuracies implied from above. 24 bits is 1 in 16.77 million or 0.06 ppm of full scale.
Hence, using \$2^{24+1}\$ in your calculations is pointless because the basic errors due to INL alone are equivalent to 1 in 200,000 or a shade under than 18 bits. If you factored in the uncalibrated gain error, it's equivalent to 25 ppm at half scale and that is 1 in 40,000 or a little less than 16 bits.
So, don't bother using 24 bits in that formula because it misses the point; realize that choosing a reference with adequate output impedance has no point beyond an accuracy of 18 bits.
Bottom line: the ADR435 output impedance is not going to be a problem.