Kudos to you for reaching to one of the best sources! Interestingly enough, the construction "es" used to be present in older French (until ca. XVIII century)

Kudos to you for reaching to one of the best sources! Interestingly enough, the construction "es" used to be present in older French (until ca. XVIII century) and then it transformed into "é". There are more of such transformations where a vowel + "s" changed into something else. A well-known one is os->ô like in hospital->hôpital (and the "os" is still visible in "hospitalité")