To be fair, the Sony Xperia XA2 is available in 5 versions. Three of them are single-SIM and two are dual-SIM models.
In Canada and USA the single-SIM variant is obligatory. Here in Europe, the single and dual SIM model is offered, although it is only a so-called hybrid dual SIM system.
But you are right - the nuances of A/B slot devices aren’t mentioned in the official installation instructions - neither with /e/ documentation nor with LineageOS Wiki.
Already in Device Information or Info about pioneer there is no mention of slot A / B phones at all.
This is the same with Samsung models. Only the model names SM-G930FD, SM-G935FD, SM-G960F/DS and SM-G965F/DS indicate that they are Dual-SIM devices. I wonder if the topic Slot A / B devices will be installed in the upcoming models SM-G950FD and SM-G955FD?