Hi Marcus,

The What Hi-Fi article contains absolutely nothing that anyone needs to know, which is about par for their course....

HDMI audio is sent from the source to the display as part of the same bitstream as the video. The video part of the stream is mandatory, the audio part is optional. Therefore you can take the HDMI stream from a player and split it into two - 1 with the video that your TV can handle and one with the audio that your AVR can handle.

The audio out MUST have video, so the splitter will generate video of a black screen to interleave the audio with. To make life simple this is done at a low resolution for the video so it can be sent under a lower HDMI / HDCP standard. This is how a 2.1 splitter can feed 8K video to the TV as 2.1 and still send HD audio to a 1.4 AVR
Ignoring shields and earths, this all takes place over a single conductor in each cable in a single direction. the player/streamer sends, the TV and AVR receive.

ARC is physically different. A spare conductor in the HDMI cable is used to send audio only from the TV back down the cable. The HD621 doesn't have any ability to connect this conductor to its audio outputs, let alone negotiate content. The HD722 can connect this conductor, but it looks like Meridian / Trinnov have set the EDID for the HD722 to say "send only stereo PCM". There is NO video component to the ARC return stream.
Being a single conductor, and a low bit-rate one at that, the capability of the reverse flow ARC is the same as TOSLINK or SPDIF. The difference is that ARC demands CEC control of your AVR, so a single remote can be used, and ARC is governed by the EDID of the components, so what you actually get is down to their negotiations. Also licensing kicks in and not all formats that are physically possible are allowed.
Bottom line ARC = stereo PCM or lossy streamed 5.1, exactly like a toslink or SPDIF. You're 100% better off with a toslink or SPDIF from the TV to the processor. Your TV should then allow you to select what goes out of the Toslink rather than relying on a negotiation of EDID.

eARC is again physically different. More "spare" conductors are abused to increase the reverse-flow bandwidth. Neither HD621 Nor HD722 have a clue about this standard. Nor do they have any ability to connect those additional conductors to their outputs. However an eARC TV will fall back to ARC in the case of an ARC receiver, so an eARC TV will still send ARC to the HD722. In Stereo PCM.....
eARC doesn't use EDID, it has its own ability-coding string, the contents of which are secret unless you are an HDMI licensee... So even if you do connect an eARC Tv to an eARC AVR, it's a different lottery as to what audio standard ends up getting sent up to the TV and back to the AVR. Added to this is the amount of jiggery-pokery to be performed by the pathetic processors in TVs, which is widely reputed to sometimes cause horrible lipsync issues with eARC audio. YMMV....

What you have to remember is that both ARC and eARC are principally designed as convenience measures for cable-phobes and as "keep it in the HDMI/HDCP eco-system" approaches. They are absolutely NOT designed to produce the best possible audio experience. eARC is a sticking plaster for "Atmos" soundbars, not a window on a whole new world of quality audio....

If you want the best audio (and video and UX) experience, use external players and streamers (even an Amazon FireTV stick is better than a TV App) and split the audio in the forward flow to the HD621. Ignore ARC and eARC.

Hope this helps,

Julian