The conversion is meant to avoid the expensive backreference optimization when it has been already done once by Zstandard, because you can't prepend a Zstandard

The conversion is meant to avoid the expensive backreference optimization when it has been already done once by Zstandard, because you can't prepend a Zstandard bitstream with a Brotli bitstream without turning one to another. But well, I think such hybrid scheme is hard to make when the latency is at stake.