how can a NUC decode audio same then a 1k AV.
Very easily. If you extract the same .rar archive on a $10000 computer and on a $100 computer - the extracted content of the archive will be bit-identical on both PCs.
Same goes with lossless audio formats.
Those HBR audio formats like DTS-HD MA, Dolby TrueHD or flac are merely a lossless compression of data. It makes no difference where the decompression takes place. Bitstreaming and decoding to LPCM are just 2 different methods of delivering the same audio data to the AVR.
Bitstreaming makes sense on those devices that for whatever reason cannot decode the above mentioned formats. Since Kodi can do a lossless decode of any HBR format - the absence of bitstreaming should never be an issue.