I can explain that part, here’s a quote from the relevant post:
Before that I had ran the lock_critical manually after having already booted the stock Android installation. I didn’t run the fastboot lock command but instead started the iodé installer at this point because it required an unlocked but locked critical state.
Fairphone OS, and most other custom ROMs (as far as I’m aware), will reset get_unlock_ability on first boot.
A new user who unlocks the bootloader, flashes factory images for good measure (which boot by default after installation), will automatically end up with get_unlock_ability at 0. Iodé not resetting OEM unlocking doesn’t mean it gets enabled again, if it’s off it’s off.
With iodé you can just turn it back on again, even with an unlocked bootloader (usually greyed out in that case on other ROMs), but since FRP got triggered that’s not possible anymore.
(Edit: Now that I’ve thought about it a bit more, that really shouldn’t be happening at all. I know for a fact, that installing FPOS again will turn get_unlock_ability to 1 again, without the user having to touch the OEM unlocking switch (assuming the bootloader wasn’t locked of course).
So the question is, does installing iodé behave the same. If it doesn’t, what’s the reason for that, could it be related to not flashing FRP.
I remember that we tried flashing FRP when people started bricking their FP4s years ago to get get_unlock_ability back, but I don’t remember the outcome, nor do I have a spare phone to test… 
)
If the user unlocks the bootloader and then directly installs iodé without flashing factory images first, get_unlock_ability stays at 1, because the first boot on iodé doesn’t reset it.
At that point the bootloader could get unlocked again, and the FRP blocker could be solved by flashing factory images, same as the other thread.
Now the actual issue is iodé not flashing the FRP partition. There’s no reason to keep a Google account in FRP, there’s no mechanism to disable it from microG, it serves no purpose on a phone without Play Services, other than create a potential brick risk.
My proposed solution for this specific case was: We need an ota that flashes FRP, if that is possible, otherwise it has to go to Cordon to get reflashed.
And in the future, please start flashing FRP during first install 