not directly from the debian project no, you can put a generic ARM64 debian rootfs on it, but you'll need the kernel and u-boot specifically built for it. There is pinephone support in mainline linux but it'll probably 5.8 or 5.9 before you can really use it on the phone.
yes the DTB is in 5.7, but if you compare the dtb file merged there and the one in the kernel fork that's being worked on you'll notice that a lot of stuff is not in mainline yet, so those components are left out. For example in 5.7 it doesn't support display yet.
A patch has already been accepted in linux-next for the driver for the display and will be in 5.8, ther are a bunch more components that are missing with a pure mainline kernel.
