> Learning and mastering a new language and applying it to an existing code base is quite a lot of work. That takes a lot of time away from other things they might be working on.

Nobody is asking them to do that. They are being asked to review C patches to enable Rust-in-Linux.

> I think these are mostly (overwhelmingly?) volunteers, so you can't just assign them work

The vast majority are professionals. A few work for the Linux Foundation, many work full time on Linux inside of big corps. For example, Ted T'so works for Google.