> Sounds like if you so much as use a data structure defined by WordPress, then by distributing your software you're really distributing a modified version of WordPress.

Given that reimplementing an API for interoperability is at least sometimes not a violation of copyright (see, e.g., Oracle v. Google), using a datastructure defined in a piece of software providing a plugin API to interoperate with it cannot make the resulting software exist only as a combination with the software providing the API, since it relies only on some software providing the same API, not necessarily the particular software that originally provided the API.