That doesn't appear to be true:

https://cran.r-project.org/web/packages/brotli/vignettes/bro...

I'm sure you can use those results to argue that LZMA is superior in some way (e.g. compression speed) but it definitely isn't clear cut superior in other important ways (compressed size and decompression speed are inferior).

I can see why, given those results, that they would use Brotli over LZMA.