Not sure. Might be political tension. Might be that phishing attacks are typically user error, and Signal feels like at a certain point it's not their responsibility. Hard to say beyond conjecture, and I didn't see a clear reason given in the article.
The article explains everything. The gist is Russians are targeting Ukrainians with phishing attacks via Signal. There also is the suggestion they're exploiting the linked devices functionality, though I'm not sure how.
It's also full of unapologetic Linux users.