That's a HDMI.org group issue more than anything else. When LTT tests their cables, they are testing up to the full 48Gb/s that HDMI 2.1 allows.
What the HDMI group allows is completely assinine and stupid IMO. The same is true of what the USB group allows and their naming conventions.
HDMI eARC is a HDMI 2.1 feature, and requires no upgrade, that I'm actually aware of, in terms of data rates. So, you can use eARC (HDMI 2.1), with standard HDMI 2.0 cables. There are a host of other HDMI 2.1 features that require no additional bandwidth as well. It's kinda crazy. There are only some specific resolutions that fully saturate 48Gb/s cables, and almost nothing actually supports that. Displays may max out at 40Gb/s or less. Game consoles (PS/XBox) don't reach that 48Gb/s mark. So, we are just hearing so many people talk about HDMI 2.1 who don't realize how silly it all is right now.
Get a 4K cable, call it a day. Unless you're an avid gamer, in which case, make sure you're working with displays and equipment that fully support every aspect of what you are doing. Then just get a certified 8K cable that's 6 feet long and it almost definitely won't have any issues at all.