Thanks for confirming. I was more responding to the title claim, which seems incorrect - the desktop site option clearly still works for others. It may be a bug, or a new policy but time will tell.
I found that you have to enable desktop mode before entering slack on mobile. If you have visited slack with a mobile user agent, it will force you to download the app even if you enable desktop mode.
yeah - I read their response at the time, but it occurred to me it may have been a response in haste or a misreading (it's the desktop option on mobile browsers; mobile browsing per se isn't supported for a long time I think), or a pragmatic implied "until we've investigated" as it worked for me on testing, and others, and its probably a skeleton support crew over Xmas.
Otherwise it appears to be premature or unresearched, or Slack have quickly backpedalled or fixed a bug.
There are reports in the Twitter thread of mobile browser access (using show desktop site), and I'm having no problems on mine.