My husband and I were on cape cod, and he had a bike friday with an old shimano 7 speed set up. His sti rear shifter failed, and we couldn't find anything in the local bike shops, and had a trip to P-town planned. I happened to have a set of new 9 speed brifters I had just picked up for another bike I was planning to build when I got home, so we decided to try it. We were also surprised, but it really did work fine. Perhaps it doesn't matter if the spacing is different, since you use the limit screws to tell the derailleur how far to move with each shift.
I think it is fabulous that it works, really! I am just saying it is not common for it to work, nor will it likely work on an 8-speed cassette. The issue is the amount of spacing between the cogs, which is different for 8 or 9 speeds,and the shifters are designed to work with one kind of spacing, tho sometimes you get lucky if you play with cable tension enough.
Limit screws actually only control how far outboard or inboard the derailleur travels. They have no effect on how far the derailleur moves with each shift.