I agree that (while the ethics of this are a different issue) the copyright question is not obviously clear-cut. Though IANAL.

As the LGPL says:

> A "work based on the Library" means either the Library or any derivative work under copyright law: that is to say, a work containing the Library or a portion of it, either verbatim or with modifications and/or translated straightforwardly into another language. (Hereinafter, translation is included without limitation in the term "modification".)

Is v7.0.0 a [derivative work](https://en.wikipedia.org/wiki/Derivative_work)? It seems to depend on the details of the source code (implementing the same API is not copyright infringement).