Using a maintained and up-to-date browser is a reasonable requirement for an IT department (should be for anyone really). Would you suggest they should be allowing IE6 just because a user might prefer it?
Of course Google is going to suggest using Chrome, if they detect that the browser might be out of date.
We don't know. The author doesn't mention how current the Firefox browser is/was.
If the organization is indeed enabling a specific check for Chrome that seems a little over the top but they're the ones supporting their users and if they want to make their life easier by only dealing with one browser that's their decision to make. It's like saying that everyone has to use Windows, or a specific line of laptops, or any other standardization to simplify the support workload.
It's not clear to me that Context-Aware Access is as configurable as you're implying. At a glance, the docs seem to suggest that Chrome is the only browser you can force standardization on, which IMO does push this towards being Google's fault.
That's correct, there is no way to say "only allow Firefox" in CAA because the attestations are either browser agnostic or chrome specific (as part of the managed Chrome offering that GSuite supports).
If we are meant to believe that this is a Chrome-invasion-move, it's the least effective lever of all times. Most of the time the more plausible explanations are just the likely ones.
Chrome was created because Google felt that the IE monopoly was hindering the advancement of web standards and improved browser capabilities. I suppose you could argue that was a different Google at a different time, but at one point they did feel that browser diversity was a good thing.
I mean, they claimed to be for browser diversity when it was not them on top lol. Underdogs want the race to tighten up, 85% market leaders want to stay out in front.
If that's a the goal, then IT department should start by blocking user ability to install Firefox or other unapproved software not by blocking access to google workspace. Blocking access to google workspace using Firefox doesn't prevent using it for everything else. It's not like the google services are going to exploit a vulnerability in Firefox, everything else might.
Of course Google is going to suggest using Chrome, if they detect that the browser might be out of date.