Well, those are both 8 and 9 speed shifters only because they are compatible with two different shift ratio RD's. The 7400 RD has a shift ratio of 1.9 and any other semi-modern shimano RD (excluding 11 speed which is around 1.37) has a shift ratio of 1.7. The 2.5mm cable pull just happens to work out as 8 speed cassette spacing via a 7400 RD, but would be 9 speed cassette spacing with a modern shimano RD such as a 6500 ultegra.
If you tried to use a 6500 RD with those shifters and shift 8 speed, the results would be pretty lousy as you'd be under shifting by .25mm each shift. Even if centered on one of the middle cogs you'd be off a full mm at the end of the cassette which more than likely would be unacceptable. I recall having tried between 8 and 7 with a modern shimano RD and that was not acceptable either using 8 speed shifters.