Canon EF 100-400mm f/4.5-5.6 USM L IS

Was reading review of the lens that I used that day.

Something came to me:

One of the primary selling argument is the IS (Image Stabilizer). It is a 1st generation variant (or v1.5 depending how you look at it) which gives you the equivalent of two extra f-stops in handholdability - at cost of shutter speed with all the related side effects regarding subject and/or background movements. On the EOS 350D with a max. 640mm equivalent this translates to a min. shutter speed of 1/160sec which roughly matches with my field experience. So far I was never able to achieve critically sharp results on a reliable basis beyond Canon´s claims. Others may be more successful here.

Then I must be very lucky, because inside the auditorium, even when using iso1600 and max aperture and an exposure conpensation of -2/3EV, my shutter speed is only on the range of 1/10 to 1/30. I did screw many photos at 400mm, but some of the photos are still quite usable.

Is it because I am using my non-IS kit lens all the time?