There are a few wrong assumptions in that statement.
ARC or eARC are ways of getting a sound signal out of a display (e.g. out of your projector) and into an AV amplifier of some kind. If the display itself is the source of the sound signal - e.g. if you are using a built-in app on a Smart TV - then you need some way to get the sound out of the TV. eARC is how you do that.
But if you’re proposing to use an OSMC device as the source of your video and sound, then you probably don’t need eARC. The most common way to connect everything is to connect the OSMC device directly to the AV amp, and then connect the amp to the display. (AV amps generally have multiple inputs, so you can connect multiple devices to them and use the AV amp to select which video signal gets sent to the display).
Some people do use eARC so they can have a single cable running from the TV to the amp, and then connect multiple devices to different inputs on the television, and use the television to switch between them (so you’re using the display to switch between sources rather than using the amp to switch). But even in this case, you don’t need HDMI 2.1 to get the signal from the source to the display. Even if one of the source devices was only HDMI 1.2, the display would still be capable of sending the audio from that source via eARC.
The only reason you need HDMI 2.1 is if the video signal you want to send from the source to the display requires very high bandwidth. You can send 4K 10- or 12-bit HDR at 60Hz over HDMI 2.0. HDMI 2.1 is only used for things like 8K video, or 4K gaming with a 120Hz frame rate. Nothing an OSMC device can produce requires that much bandwidth.