alt Hacker NewsClean room is sufficient, but not necessary to avoid the accusations of license violation.
a2mark has to demonstrate that v7 is "a work containing the v6 or a portion of it, either verbatim or with modifications and/or translated straightforwardly into another language", which is different from demanding a clean-room reimplementation.
Theoretically, the existence of a publicly available commit that is half v6 code and half v7 can be used to show that this part of v7 code has been infected by LGPL and must thus infect the rest of v7, but that's IMO going against the spirit of the [L]GPL.
Please don't use loaded terms like "infect". The license does not infect, it has provisions and requirements. If you want to interact with it, you either accept them or don't use the project. In this case, the author of v7 is trying to steal the copyrighted work of other authors by re-licensing it illegally.