Yes, I can. Manufacturers literally CAN NOT closed-source fork the kernel, because of its licensing. They HAVE to release the exact kernel sources used for the device.
You can also run fully open Linux on your Pi5. There will be some firmware blobs, but then again so will any x86 board.
Oh, and smartphones run ANDROID. Not just Linux, but a completely separate layer on top of it that is much different from the traditional desktop environments. Android isn't Linux, it runs on top of the Linux kernel. Sure manufacturers don't release their Android source, because they don't have to, but equating Android with Linux is idiotic.
You seem to have a very... loose grasp as to what is considered open source and what "Linux" is.
Here's a quick bootcamp: any fork of the Linux kernel is still Linux. Does the RasPi5 require a custom kernel? Yes. Does that make it any less open? No. Does that make it any less Linux? Also no.
The kernel sources for Raspberry Pi boards are readily available. No request for access needed. You can clone the repo any time. That is open source.
Samsung, Google, Motorola, Xiaomi, OnePlus, Nothing, etc. all either provide open repos for kernel sources, or provide them upon request (as per the GPLv2 license of the kernel). That's still open source. And just because each phone manufacturer - as well as each SoC manufacturer - had to make changes that aren't in mainline, doesn't make that any less open or any less Linux.
In fact Google has been pioneering a way to get better kernel baseline support via GKIs, and manufacturer support for a lot of boards and SoCs have been introduced in the mainline kernel.
But the thing is that ARM is progressing fast, with quite a few SoC/core manufacturers on the market (Qualcomm, Apple, Samsung, HiSilicon/Huawei, MediaTek, Amlogic, Allwinner, Broadcom, Rockchip, NXP, UNISOC, and so on), whereas X86 has two manufacturers who do end to end development in-house, and at a slower pace (usually one class of CPUs announced a year). The thing is, mainline WILL pay behind because ARM just progresses so fast that reviewing all the changes to add a new SoC to the supported list cannot happen overnight. That's why forks exist.
But, I repeat, that fork makes it no less Linux nor any less open source.