"Friends and major donors insist that Mrs. Clinton is sincere in expressing ambivalence about seeking the presidency again..."

"... and they go so far as to assert that she is simply happy to have time to clean out her closets for the first time in decades."

Sincere... Clinton sincere... depends on what the meaning of sincere is.

The releveant OED definition of "sincere" is "Characterized by the absence of all dissimulation or pretence; honest, straightforward." Referring to persons, the word in English goes back to 1539, and the (unlinkable) OED has a lot of historican examples, but one stands out:

1824   Byron Don Juan: Canto XVI xcvii. 112   For surely they're sincerest, Who are strongly acted on by what is nearest.

In context:

So well she acted all and every part
By turns — with that vivacious versatility,
Which many people take for want of heart.
They err — 't is merely what is call'd mobility,
A thing of temperament and not of art,
Though seeming so, from its supposed facility;
And false — though true; for surely they're sincerest
Who are strongly acted on by what is nearest.

Right now, the closets are most near/To clean them must be quite sincere.