Admirals are meant to be even with one another. No one believes that one can kill the other without simultaneously dying themselves.
Look at how oda portrays Sakazuki compared to Lizaru and tell me you think it’s similar.
That’s not how that works, admiral is just a label to the strongest marine fighter. It is not out of expectations that they can beat each other in an all-out fight.
This is like saying that a professional boxer beating another pro boxer would be shocking. Admirals and yonkos are labels to the strongest fighters, and who can realistically beat the strongest fighters? — The strongest fighters.
It will only be shocking if it was reported that it happened with little difficulty.