The source code for baseband software needs to be open source, I expect there are many more such vulnerabilities. I also wonder how many basebands contain Linux and are GPL violating.
But yep, would be nice if it was open source, although not sure how much that would help (only if sufficiently motivated auditors can be bothered to look at it). A bunch of baseband firmware is even encrypted on disk now (loaded into BB memory from the kernel)
Load that into Hexagoon IDA plugin and you'll see it's bog standard Hexagon for all the remote GSM/LT code that actually does stuff (similar to the project zero research). I haven't verified (and don't own a Pinephone) but most Quectel boards I've seen in the past do enforce signature validation, so binary patches are not easy.