Is the fast of one who ate, assuming the time of Maghrib had begun, then it became clear to him that it had not, valid?

"If someone breaks their fast, thinking the sun has set, but then it becomes clear that it has not, then if the fast was obligatory (fard), they must make up that day according to the majority of scholars, and this is the relied-upon opinion (mufta bihi). However, if it was a voluntary fast (tatawwu'), some scholars say they are not required to make it up. Sheikh Al-Islam Ibn Taymiyyah is of the view that their fast is valid and no make-up is required, citing the hadith of Asma bint Abi Bakr as evidence."