The 10 GB Ethernet standard predates the invention of 2.5 and 5 GB Ethernet so 10 GB devices based on older revisions of the standard will never negotiate to 2.5 or 5 GB speed – they don’t know about it. They will however negotiate on 1 GB Ethernet, but this is not enough for 802.11ax ( Wi-Fi 6). Also there were competing and – as far as I understand it – incompatible first implementations of 2.5 and 5 GB Ethernet (see for example this article).
While in the meantime 2.5 and 5 GB Ethernet got standardized and integrated/amended to existing standards as well, it seems to me that when it comes down to compatibility – especially for CE devices – it’s left to the users to figure out working combinations of devices that are able to actually communicate with each other with 2.5 GB/s or 5GB/s speeds. As so often before, it seems that the only important thing is that the marketing department can put a feature on the plus side in order to hold up prices.
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