They were infront of people eyes...
They just chose to:
1- ignore it
2- to bash any theory regarding that, and call whoever said it "dumb" or "farfetched" or whatever just to shut down the idea of seeing Oden in full action in this war...
People don't realize that for the amount you hyped Oden so much... you basically limited EVERYTHING from his skills, Oda basically didn't show even a fraction of his strength.
- We hear his clash with WB was intense, but only see one panel and cut off...
- We hear about his King's haki BUT NEVER see him use it
- We see his fight against Roger pirates skipped, not even againts a commander or rayleigh was shown
- We ended up believing that Oda didn't want to spoil much about Roger, but he will at least show something with Kaido
- NOPE, nothing, we will only see the cut panel, and the last hit by Kaido.... nothing else...
Why would you hide SO MUCH about a character when you intend to kill him in his flashback... the only answer is.. you will show more of the person in future... as simple as that