That quote doesn't mean they always use Teams, they may have clients that only use teams and so are forced to comply to client.

I would hope internally they have a Jitsi server

I don't think it necessarily means anything. Teams is very well-known.

It's less about using something proprietary as much as using something so excrementally terrible as Teams.

Somewhat related - who came up with the idea of stuffing all that domain verification tokens directly into TXT records for the domain?

Just querying the TXT record of a domain might give you an idea what products a company is using...

We'll give you three minutes back, as they say on Teams meetings!

I don't think this is enough evidence to show they use Teams. At work we say "zoom calls" and use Google Meet. It could very well be someone's catch-all term for video call meetings in general

First, Teams works well on Linux. I have been a desktop Linux user since the 90’s and I use Teams every day (week days at least).

Second, that does not mean they use Teams as their preferred collaboration software.

Even on Windows, you use what the meeting organizer used to schedule the meeting. And if you interact with external companies, you are going to be joining Teams meetings regardless of your preferences.

And, if you had to make a reference you thought everybody would get, Teams or Zoom seem like your best bet.

So making reference to something someone one would say in Teams is not exactly Ronald McDonald admitting he eats at Wendy’s.

If Teams IS their preferred solution, I think the bigger deal may be a European company relying on a US cloud provider, even more than proprietary vs FOSS. At least, that is my view.

I would love a great Open Source video conferencing option to emerge and become popular though. As above, this kind of software has network effects and I would rather get invited to Open Source meetings if possible.

I have had calls with SUSE sales reps because I'm in the enterprise space, can confirm they use Google meet and Google workspace in general. Still not FOSS, but not Microsoft.