Sorry for asking a naive question: how do I restore an Xperia X running Sailfish X back into Android?

All guides I found online assume it is running Android to start with. No one ever expects starting from running anything else.

Nevertheless, I tried running Emma. As per this guide, I turned the phone off, waited 30 seconds, plugged the phone in the USB while holding the Volume Down button. The indicator LED lit up green (VERY dimly, barely noticeable) but nothing else happened. Emma does not recognize the phone.

To preempt some questions, I also tried Volume Up, as well as starting Emma before or after plugging the phone in. All with the same result.

Caveat! It might be relevant. My phone is in the "battery only charges up to 20%" mode. I had it once but don't remember how I got out of it. I cannot get out of it now.