My understanding is that Apple (claim to have) optimised their implementation for energy efficiency, and feel that they would get the blame if using e.g. Chrome

My understanding is that Apple (claim to have) optimised their implementation for energy efficiency, and feel that they would get the blame if using e.g. Chrome resulted in lower iPhone battery life, because users wouldn’t realise it was due to Chrome — all they’d know would be that they weren’t getting the advertised battery life.