It could be because they reasoned several pellets hitting and flattening themselves rapidly against the plastic centre part are more likely to transfer a greater amount of energy particularly since as mentioned the lower antimony retains more energy downrange. In other words there is a different requirement to score a kill, clays need only produce a fracture which will lead to the break up of the target within milliseconds, Helice needs the centre clean knocked out.
Incidentally it's not proven beyond doubt that softer lead cannot produce perfectly tight patterns, the Americans load their own as we know and they suggest the differences in density can be pretty tiny so long as the shot is contained within a quality plastic wad and the propellant/primer well matched. In fact at close range it is apparently very difficult to produce scatter characteristics using poor quality shot without using the appropriate wads as well.
To me the reason high antimony and high end shells go hand in hand is more to do with the manufacturers finding it easier to maintain spherical perfection throughout the manufacturing process when using better quality shot. No one is going to buy 5% antimony loaded into a slowish budget looking package with a £220 price tag. I have always found shell speed to be a huge contributory factor in delivering uber impressive kills, the more impressive the kill the higher the shooter confidence and the easier it becomes to remember what you did to produce that result so repetition becomes easier.
The other thing I notice when shooting cheaper (and presumably lower antimony) shells is that smokes go from vapour to crumbly at anything much over 30 yards, this has to be due to harder shot finding it easier to punch/damage/fracture a brittle object than the admittedly still capable 2% stuff.
