Originally Posted by
zymphad
Normally yes, but as others said, this was more than just length change and not sure why you don't believe Shimano when they list the compatibiity and pro bikes were seen using 9001 series shifters and using 9100 FD/RD/Brakes, while they waited for delivery of the rest of the groupset.
You change the mechanics of the lever-arm and nothing in the shifters....then not using the 9100 brifters should (reasonably) result in a much harder to shift front mech (either up or down, depending on if you change the spring tension as well). Which is why the other 99% of arm-changes of the FD Shimano/Campag/etc deem prior year shifters not compatible.
It is, well, physics. I'm skeptical because it honestly sounds like a typo or oversight.