I've done some digging and:
---My Samsung TV can pass Dolby Digital 5.1 via HDMI ARC
The claim that Samsung makes about ARC and SPDIF supporting 5.1 audio is technically correct. The TV is capable of outputting 5.1 audio through ARC and SPDIF. However, they are not very forthcoming about the fact that it will only output 5.1 audio through ARC and SPDIF when the TV is on Cable/Air mode or through their native apps (as the OP noted). Based on the info above, you should be able to put together why their native apps pass 5.1 audio without me explaining that here. All other modes of the TV (when HDMI is the input source) will output 2.0 audio from ARC and SPDIF.