“Tous les mains” is indeed invalid as a unit (since “mains” is feminine), but it is nonetheless a valid sequence of words in this sentence because it is not being used as a unit. “Tous” is an pronoun, not an adjective here. It is not connected to the following noun phrase; rather, it refers back to the subject “ils” (with which it agrees).

This may become more apparent if you try to translate the text to English: “They raised all their hands to heaven” doesn’t make as much sense as “They all raised their hands to heaven…/All of them raised their hands to heaven…”

Note that the pronoun “tous” is pronounced /tus/, with the consonant sound /s/ at the end, unlike the adjective “tous” which is /tu/ before consonants and /tu.z/ before vowels.

Here is a thread on the Wordreference forums that you may find helpful: FR: tout / toute / tous / toutes

tous here applies to ils, not les mains. Therefore, it is perfectly valid.

In English you would phrase it as They were all raising their hands

You can trust Voltaire with his French, it was top notch!