No, I'm sure it was ice mode. I hit ABS all the time on track, even on full slicks. It has a very specific pedal feel. ABS in this car is very high frequency and you can feel a buzzing, but the pedal feel is fairly normal. Ice mode makes the pedal feel rock hard, and it doesn't depress as far - it's like there's something between the pedal and the floor.
I know roughly how much grip I had based on how hard I was able to turn, and it was way more than my brakes were letting me use. There's another turn on that track where the rear inside tire gets real light, and I can hit ice mode 100% of the time, and actually have to be careful there every lap.
It's a bad system unless you're driving on ice
I've driven many cars at the track, and let me tell you, the Elise is the only car that I've encountered so far with this strange braking behavior. I like ABS enough that I'm not willing to sacrifice it by pulling the fuse, so I just deal with the ice mode.
ABS does not just control tire slip to a certain amount - it also has to balance that while controlling to a specific rate of decel as well. So if your target slip is 10% but the tire is currently at 2% and got there very quickly then ABS may still kick in and pretty aggressively. In that case, it kicks in very hard because it knows that if it doesn't then you'll exceed that 10% target in no time.
To make matters worse, you mention tires that have hugely different grip profiles than factory, and it wouldn't be unreasonable to assume that your wheels, pads, and rotors are all different, too. ABS control is already a very unstable system, so surely it can't make that any better when you change all the physical components and then ask it to still operate perfectly.
So to reiterate:
1) ABS control is a very, very finicky procedure (think of trying to balance a broom upright in your palm. In a hurricane. Blindfolded)
2) You've changed at least two major components - road surface and tires - and never told the ABS controller how to deal with those changes.
3) ABS does not control to a slip amount only. It also has a slip rate target and will pick the one of those two that is worse. This will be exacerbated by bitey pads or lower inertia wheels/tires/brakes.
4) The ABS controller only has wheelspeed sensors as inputs, so it cannot know what is going on around it other than what can be gleaned by those sensors and the brake pressure.
There is also heavy legal pressure to intervene too much rather than not enough. Put yourself in those shoes and you'll see that this "Ice Mode" is a natural consequence of these points.