Actually I also disagree.
I think you’re mixing up things here.
You can buy murena FP5 and get 5 years of extended warranty and you can buy FPOS FP5 and you get 5 years of extended warranty.
You just can’t buy FPOS FP5, install /e/OS and claim the warranty with this changed OS installed. And I think that’s logic as FP can’t support their HW with different OS versions installed.
And AFAIK it has always been the case that there were closed source parts of the OS (mainly the driver/modem parts) and open source parts (the adapted AOSP part). I think that’s the same for FP3, 4 and 5.
I think they currently even stretch a little bit into alternative OS region by selling Murena FP5 on the FP shop page.
…and the noticed that as a small company they can’t afford this development. So they released the sources to give the community the possibility to improve these apps. You can find discussions and reasoning if you read the according old topics on this forum.