Victa serpente is not to be interpreted/analyzed here as an "Ablative Absolute" but rather as a dominant participle construction (see below for a definition) depending on an adjective superbus, which takes a complement in ablative case (NB: so-called "Ablative Absolutes" act as adverbial subordinate clauses, whereby they do not depend on adjectives but are adjuncts to sentences).
So-called "dominant participle" constructions are often referred to as Ab Urbe Condita (AUC) constructions, where the participle can often be translated into English/Romance via a nominalization: e.g., ab urbe condita 'since the foundation of the city/Rome'; de Milone per vim expulso queri 'to complain about Milo's violent expulsion', absoluto Scaevola gaudere 'to be delighted by Scaevola’s acquittal', victa serpente superbus 'glorying in the defeat of the snake'; etc. NB: importantly, as you can see, dominant participle constructions can depend on a preposition, on a verb, or on an adjective (as in the present question: superbus).
"Dominant participle" constructions (aka. AUC constructions) are defined by Panhuis (2006: 172) as follows: "syntactically speaking, an attributive participle modifies its head noun. But as far as content is concerned, the participle may express the leading idea and thus be the dominant element in the phrase" (Dirk Panhuis (2006). Latin Grammar. Section 363: "Dominant participle").
Basically, the idea of "dominant" participle comes from the fact that the participle in an AUC cannot be omitted but it is compulsory. Otherwise, the construction is ruled out or its meaning is changed. Notice that the meaning of ab urbe condita (on the AUC reading) is quite different from that of ab urbe: semantically speaking, the complement of the preposition is an entity in the latter, but a state of affairs or a situation in the former.