It's probably quite hard to explain at the microbiological level for sure, but you don't need to be able to explain it to understand how it will impact outcomes

It's probably quite hard to explain at the microbiological level for sure, but you don't need to be able to explain it to understand how it will impact outcomes. You just need to be able to measure it. Pre-existing immunity is by definition immunity that existed pre-exposure, immunity that comes from already being infected before was referred to as natural immunity. The latter was accepted as real right up until the vaccines arrived. The former never was, I guess it still isn't as this is just a university press release, not something formal from public health agencies.