Crossing dips with tets and that it rarely works is covered in a few places, e.g. this post from Maurice:
http://garden.org/thread/view_...
But in your example above, both parents are tets. 'Buffalo Thunder' is registered as a tet:
http://www.daylilies.org/Dayli...
While the original 'Rose F. Kennedy' is registered as a dip, the "tetra" in front of the name indicates that it was converted to tetraploid and that the tetraploid version was used in the cross. So it is tet to tet. The AHS doesn't list tetraploid conversions separately from the original diploid.
Edit - obviously I took too long to type and ended up cross-posting (can one not include an emoji in an edit?)
