that line was explained in the line in the revision proposal

Before calling something an intrusion and doing a commit, one would expect to have at least read the differential revision proposal to see the reason why the code was written the way it was written.

i know that people here like to speak with parabolas,

This is not a parabola but an instruction where to find information .

it would have been quicker to tell

It would have been quicker for you to click the link in the commit message rather than asking me to reiterate its contents.
If you refuse to read the link pasted in the commit message and in the answer as to why the code was written this way, I can paste it here:

From the summary of D595:
From the comments:

@mimo I suspect you might want to use this feature and I suspect others don't trust me that I can find oversights on my own

The line removed in this commit was added in version Diff 4437 of D595.

From the patch upload notice:

Developer overlay player switching support

From the inline comments specifically from the line removed in this commit:

relevant in case of switching the perspective to observer using the developer overlay

I suppose if you are bothered by the UI enforcing a gamespeed according to the player assignments then you should have also removed the other updateGameSpeedControl call playersFinished.

What has the preference, the developer overlay working correctly or people sending random commands via F9?

Why don't the people sending random commands just send the correct commands or extend the code to support what they want to achieve or use the UI that already supports what they want to achieve?

In fact fast forwarding for people using the developer overlay was a feature correctly implemented in the GUI until this commit broke it while calling it intrusive and bothersome while refusing to read or investigate the reasons for introducing it.

Reproduce:

Step 1: Start SP game as player
Expected behavior 1: You cannot fast forward, because you are a player.
Step 2: Alt+D to open the developer overlay
Step 3: Click on change perspective
Step 4: Select the observer slot
Expected behavior 2: You can fast forward, because you are an observer.
Step 5: Disable change perspective feature:
Expected behavior 3: Because you are an observer, you can view different players perspectives without becoming assigned to that playerslot and can still fast forward.
Expected behavior 3 was broken by this commit as it now prevents you to fast forward as an observer.

Unless you want to be able to send simulation commands while fast forwarding, your use case was implemented in the GUI but you used the GUI incorrectly based on not understanding what the change perspective developer option does.
If you do want to be able to send simulation commands while fast forwarding, then the instead there should be a developer overlay option enabling that. Perhaps thats also what the Delenda Est developre wants who enabled those speeds for players.
If you want a different speed than present in the JSON files, then the JSON file should be extended.