The 456Mbps was a test of transfer rates, not a router's broadband to a second adapter. Note some of the article's other findings:
"Next, when tested in a more realistic setup with the second adapter in a room two floors away, speed predictably dropped – here to 119Mbps."
"Our third test was on the Wi-Fi performance in the second room (far from the original wireless router). In a non-Mesh trial, we achieved 104Mbps, which isn’t so far off the wired speed"
This showed that performance deteriorated markedly when not in the same room and that WiFi was slower than wired - both as expected.
Yes speeds drop when you move the WiFi adapter away from the main adapter that’s connected to the router - fully expect that to happen. Note that the review references a “real world” test and when both adapters were in the same room they achieved 456mbps. Obviously each home will be different depending on the wiring etc. However this surely proves that homeplugs can achieve speeds in excess of 190mbps.
Appreciate that the model reviewed is higher spec than mine (2,400mbps vs 1,200mbps).