I noticed this in spoilers too, but thought it might be a mistake, but dodesn't look like it was, there seems to be a mistakes with dialouges used.
Oden is saying he was sent into future. But the reason they gave is that its impossible to return to past
.
There should only be one reason that they know he definitely died at that time. still it could be possible that oden did not die at that time due to something.