What the f*ck are you talking about? First and foremost, UEFI is actually about interoperability, and most definitely not about "shutting out third party software". Plenty of the stuff it does (e.g. searching for ESP partitions on all media instead of hardcoded, defining a portable bytecode for running firmware from extension cards, etc.) clearly shows they really didn't have security in mind, and instead they were trying to create something unified even between different ISAs (at that point, Itanium and x86; today, possibly ARM and x86).

"in the run up to its launch on desktop" most UEFI BIOSes there was certainly absolute NO worry about running other OSes, if only because most if not ALL UEFI BIOSes back then shipped with the CSM aka "old BIOS compatibility mode".

I don't understand which Jolla feature they would need to disable from UEFI. And the fact that Jolla doesn't want to disable any of the "security" features is Jolla's decision, as has been argued many times (by me -- a lot -- and others).

UEFI is actually good for interoperatibility, because at least it provides a (minimal) bootloader<->OS API, which guarantees that even a stupid hobbist like me can probably run a HelloWorld-like OS.

You are most probably confusing the concepts of UEFI and "Secure Boot", which was an idea that appeared relatively much more recently.

There was indeed a backslash more or less by the time, and that's because Microsoft, in their infinite wisdom, mandated Win8-logo ARM devices to have a locked-down UEFI bootloader.

Guess what. A locked-down bootloader has nothing to do with UEFI or x86. ARM was there much earlier. The concept of a "locked bootloader" is something that is so common on ARM that we basically assume that every device one can buy will have a locked bootloader -- or even no bootloader at all. That is a concept that is still tremendously alien for me and the reason I see everyone who argues 'locking down a bootloader improves security' with slight disdain.

Microsoft actually WENT and released a Win8 ARM device with a locked bootloader -- the Surface RT 1 & 2. I pity everyone who actually bought one of those two stupidly crapped devices instead of the much better and hackable Surface Pro. To this day, the bootloader on the RT hasn't been brocken. Fortunately, Windows RT is all but dead these days.