Slush, not ice, can build up on, especially under, the wipers due to melting from window heating. It then builds up under the blade with airflow. That is why, perhaps, Boeing using the bolt as an indication; I know that is mostly for airframe icing. The -40c also depends on the thrust setting.
I'd always been told not to use EAI at very cold temps due to heating in the intake. This can cause very small ice crystals to warm and join together into bigger, not so friendly, ice pellets. I have to assume that the engine manufacturers have spent millions U$ on testing & researching this basic phenomenon. It's been around sine JT-8's. It's not new.
There are many new things they are learning about jet engines & cold temps; ask BA 777 crews. I think they might have got this one sorted, though.