Trillian was not Mac only. I’ve never owned a Mac and used Trillian almost exclusively from 2002 until roughly 2009?? I can’t remember when the transition from IM to texting happened for me, but it was around then. When I was running Linux at home I would use Gaim, which was developed by a friend of the main Trillian guy.
Trillian was definitely part of that war. I remember the daily patches to get things working again.