If you are a 1:20 swimmer, then a 20 minute improvement in your time is a 25% improvement. I don't think there is a person on the board that wouldn't take a 25% improvement in any segment.
Sure there is. Percentage improvements are irrelevant compared to total time improvements. I'm not sure if it came across clearly but he is talking about choosing between 40 minutes improvement on the bike and 20 minutes improvement on the swim. I'm sure that 40 minutes improvement on the bike is closer to 5-10 %, not 25%, but who cares? It still puts you across the line 20 minutes sooner than the guy who swam 20 minutes faster.
Obviously, this all only works if you accept his hypothetical performance improvements. Maybe he's totally wrong. But we choose to allocate our training based off what our best guess is of the benefits. If he thinks he can get a bigger total gain by cycling those extra hours instead of swimming, then it is only logical for him to choose the cycling. You might convince him otherwise, but not, I think, by talking about percentages.