Are "prototypical" and "prototypal" different?

prototypal inheritance is a technical term used in computer programming. The adjective indicates that the programming style is based on protypes.

Prototype-based programming is a style of object-oriented programming in which behaviour reuse (known as inheritance) is performed via a process of cloning existing objects that serve as prototypes. This model can also be known as prototypal, prototype-oriented, classless, or instance-based programming. Wikipedia

prototypical inheritance would mean something different. It would indicate that the [mechanism of] inheritance was itself a prototype.

Answer

Although in everyday English, 'prototypal' and 'prototypical' are synonyms with the former being the rarely used version, in a technical sphere it is permissible to define terms to mean what you want them to mean. The example you give is one such technical use.