In the meantime, my Linux guru friend at least came up with the following command to effectively do the same, albeit in a single-threaded manner (though in my case I have two main folders, so I was able to at least saturate two cores by running one instance per folder):
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find /path/to/directory -iname \*.png -exec cwebp -lossless -m 6 {} -o {}.webp \;
That however makes the resulting images end in .png.webp rather than
just .webp so to achieve that instead there's also the following alternate command (note that it requires one to first install the "parallel" package through your package manager and/or "apt get" or the like):
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find ~/Desktop/test -iname \*.png -print0 | parallel -0 --nice 19 cwebp -lossless -m 6 {} -o {.}.webp \;
Interestingly, despite the lossless compression being set to maximum (6), the resulting file sizes are still a bit larger than what XnView MP creates when similarly set to lossless maximum compression, and this was tested with a 24bit image without transparency. So it's not quite as ideal then, but it'll at least work for me as a stop-gap solution in the meantime.