Yeah, sorry, I didn't say that very well. Single value decoding of Bijou values is of course trivial in SIMD, but the performance benefits of SIMD come from deterministic boundaries across a window. ULEB128's continuation bit is fixed position, so it's data independent. One pmovmskb gives you every boundary in the window.
replyInterleaved Bijou has no such signal (tag and payload bytes both span 0x00–0xFF), so finding the boundaries is a dependent per-value walk with no opportunities for parallelism.
There's still speculation though - if eg. most values are of 1 or 2-byte length, you can speculate that any control-valued byte is actually control. You can even do a compensation pass to try to fix some amount of mis-speculations, and then bomb out if that fails.
replyWith that, it's mostly byte-parallel (though data-dependent as I mentioned).