What that sentence actually means is "we don't want to help our competition (AMD, NVIDIA) but we don't want to re-implement the wheel".
The API and fundamental part of LLVM is its IR, LLVM-IR, which is where most optimizations happen.
From this point-of-view, LLVM is a "platform", and an extremely brittle one: every release has breaking changes to the IR, the IR is constantly evolved to support new hardware and new optimizations, etc.
When you build a tool on top of LLVM, you are buying into this rapidly changing platform.
The only proven way of using the LLVM platform competitively is to be part of its future: follow upstream closely, upstream most of your code, and actively participate in the platform evolution so that your competitors can't turn it against you.
If you keep most of your code private, following upstream gets very hard. You skip one release, and then its 10x harder, so you skip another release and stop participating in LLVM's evolution cause you'll have to wait years for changes to upstream to land on your compiler. Your competitors do what's best for them, and those can be things that are bad for you, and then you are proper screwed, cause you can't migrate away from LLVM either.
Companies like Intel and NVIDIA do this, e.g., nvcc and ISPC are stuck on LLVM 7 (~4 years old), but these companies have built huge technology stacks like CUDA or DPC++ on top of it!
Intel and NVIDIA might have enough manpower to maintain their own outdated fork of LLVM forever, but at some point it just stops being LLVM, and these companies are not really much better off than where they started.
IMO, building all your technology on top of a platform that either is or can be under your competitors control is just a really bad idea.
One would hope that these companies would realize this and contribute back and help the community as much as possible, but in practice they just don't. By the time they realize it, it's already too late.