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 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.