As BrianK90 noted so well: “That message is basically stating that you're getting a better connection and quality with the specific HDMI cable you're using.”
I think the wording of the on-screen message is somewhat unfortunate, as it doesn’t reflect a good-better-best scale. It’s just a confirmation of the suitability of the cable. The cable facilitates all the AV features. The “better” part could be seen as a comparison to a lesser rated cable.
If you have an Apple TV 4K (1st generation), then your 18 Gbps HDMI cable is fine. Higher rated cables won’t improve anything.
If you have an Apple TV 4K (2nd generation), then your 48 Gbps HDMI cable is fine. Higher rated cables do not exist.
If you have an Apple TV 4K (2nd generation), and you connect to an HDMI 2.0 or lesser port on another device, then your 18 Gbps HDMI cable is fine. Higher rated cables won’t improve anything.