The definition of tristate attribute value type states:

The default value for this value type is false unless otherwise specified.

There are only two states of this type: the "aria-checked" state and the "aria-pressed" state. Both of them have "undefined" as a default value.
The tristate type is like applicable (true/false/undefined) type, but with additional "mixed" value. Applicable type's default is "undefined".
So I suggest to change that confusing definition, which must be:

"The default value for this value type is undefined unless otherwise specified.