Postmarketos is an derivate of Alpine Linux and has nothing in common with debian. They also deliver (amongst others) an Phosh-DE. They provide also plasma mobile, sxmo etc.
The battery problem is really exact the same as with Byzantinum/Phosh 
We talk not about half an hour more or not. In the car I have an USB-C loading cable and at home in office also dedicated for the L5. So I can use it as an daily driver
I do not know many about the modem access from the main OS. But if they have to look permanently for incoming calls from the CPU and have no event (interrupt etc.) driven low-profile this would explain the problem. Or the modem has no usable low-power routines and it is normal that the ear on longer phone calls is gettig hot. After a call You can on Byzantinum and PM-OS se a greatly reduced battery energy state
But I have found that PM-OS with phosh is a wonderful replacement of Byzantinum with lots of up-to-date apps… als long as You can dismiss the not working Cam