Yes, Nokia contributed the NEON code for the DCT in libjpeg-turbo.
I haven't had a chance to do a side-by-side comparison as yet, but I suspect the SnappyCam encoder is faster for many reasons, including choice of algorithm and the way they use two multiplies (low, high) at times, and their image row-by-row nature with function call overhead in favour of code maintainability.
I was involved in some NEON work on libjpeg-turbo, and I can confirm that the image buffer management there is hell, as are some other aspects of the design. A from-scratch implementation with performance in mind should easily be quite a bit faster.
I haven't had a chance to do a side-by-side comparison as yet, but I suspect the SnappyCam encoder is faster for many reasons, including choice of algorithm and the way they use two multiplies (low, high) at times, and their image row-by-row nature with function call overhead in favour of code maintainability.