Who is lying? Sure, you can argue lazy/cheap by not designing for mobile, but as someone who does some web design and does design for mobile responsiveness, it's honestly annoying and I don't blame them.
Unsupported, with a screen not showing what it should means doesn't work, for a non tech user. It's deliberate misrepresentation. This is also called lying.
You can lie by omission. You can lie blatantly and you can lie by inference.
Sure, mobile responsive design can be frustrating. However, they are a giant company with billions in resources. They are not doing it to help users, they are doing it to force app downloads and data harvesting.
When I tried this, second half of 2024, I had to spoof my user-agent and use private mode to not get ID'd as the mobile browser who just tried to load the site.
I mean it's fine to tell people that your site doesn't work as intended, it's fine that you have to request the desktop site. And I'm not complaining about the user experience. FFS I used to surf with the Opera browser MIDlet, on my Samsung d500, 20 years ago. Having an interface that matches your screen is a luxury most people take for granted these days.
But I feel like I had to jump through multiple hoops to bypass microsoft's wish that I install their client and have it be connected in the background.