The singaling is double the transfers but the bandwidth does not double.
We've seen what ... 5 ... itterations now and people still get this wrong.
The doubling of transfers comes at the cost of latency and processing overhead, always, until the new standard matures and might match the predecessor latency.
Did dual channel DDR5 double dual channel DDR4 bandwidth? Short answer ... nope, not even close, and DDR4 4000 remains viable and trades blows with DDR5 to this day in application performance despite "losing" in sulunthetics.
Lossless scaling across 2 8x slots rocks.