The spelling was an intentional choice.
<rant>
They didn't consult me for Codable, which is why we didn't end up with Serializable, which every coder(!) in the world would have recognized for what it is. Instead we got the invented concept “coding” that is neither related to every programmer's primary activity, nor encryption, and a name that elicits the pronunciation “
-able.”
</rant>
By the same token, the normal rules of English pronunciation—such as they are—suggest that Stridable could be pronounced “strid-able.” Since most dictionaries admit both forms of words like “tuneable”/“tunable” and we did have to invent a new concept, we wanted the name to guide people to the right interpretation.