No it is not. I'm amazed that people just don't get this simple fact. The drivers were reverse engineered. ThinkPad is not an open platform. It contains some Intel and AMD stuff that is open, but you can't give credit to Lenovo for that.
I'm not giving credit to Lenovo, I'm saying that the platform is mostly open because it is based on mostly open components. In contrast to Apple's devices, where the platform is closed because it is based on undocumented components. You could say the same about pretty much any standard PC, Lenovo is just one of many vendors, I have no idea why they got singled out here.
But they do sell boxes explicitly qualified and supported to run Linux, and they do contribute to the Linux kernel development process.
The fact remains that claiming any of Apple's hardware platforms are remotely open is laughable.
"ThinkPad linux driver reverse engineered" or "lenovo linux driver reverse engineered"
Power management, I2C devices, touchpad, touchscreen, audio, WiFi, bluetooth, etc. Many things besides the GPU. Maybe some of it is open now (Intel parts) but that wasn't always the case.
As another smart person (smoldesu) pointed out here, Lenovo has recently started contributing some updates to the kernel, but the vast majority has been reverse engineered over decades.
The attitude of Linux devs was always to accept that hardware is proprietary/undocumented and get to work on reverse engineering. Then users take it for granted that stuff just works and have no appreciation of the effort that it took to get it working.