Using the tool, you get the same difference of 1.6 if the roster is Schroder, Smart, Richardson, Tatum, Williams (swapping Pritchard for Schroder gets you 1.7)
but the best lineup in that with a difference of 2.0 is Schroder, Smart, Brown, Tatum, Williams
So it seems strange they say what the optimal lineup is, but then don't actually have the optimal lineup.
All that said, is I would start Horford instead of Williams. I think that makes the most sense and will hopefully allow Williams to stay healthier.