the buttons of the dialog are defined as "role=toolbar". In this case the buttons need to be controlled by the arrow keys and to disable the tab function.
"For a horizontal toolbar (the default):
Left Arrow: Moves focus to the previous control. Optionally, focus movement may wrap from the first element to the last element.
Right Arrow: Moves focus to the next control. Optionally, focus movement may wrap from the last element to the first element."
Also the "role=toolbar" should just be set, if the item count is three or more.
"Use toolbar as a grouping element only if the group contains 3 or more controls."
You should remove the "role=toolbar" or just set it with three items or more and add the arrow control.