I'm not as familiar with UBPorts/Ubuntu Touch, but you can do this with Debian on the PinePhone, or use other package managers like Pacman with Arch/Manjaro. I think you're conflating the phone with UBPorts, which is just the preloaded OS on one edition of the phone. There's nothing Ubuntu-specific about the PinePhone.
Mobian on the pinephone does exactly that. There is nothing stopping someone from a) flashing the eMMC directly to a new distro or b) flashing it to an SD card to try it out.
I will wait until there is a phone where I can just open a terminal and type apt install whatever I want.