AFAIK they just used old SkypeKit version, which works on Win 8.1. But we don't have such binary, because when we started Skype plugin on SkypeKit, it was already in this newest version. And because SkypeKit libraries are signed and allowed to run only with original application, we can't use SkypeKit from Trillian.
And SkypeKit support will be lost either way in few months (maybe 8?), because Microsoft killed it. Right now you can use SkypeClassic plugin on Windows 8.1. But it will be perhaps dead in same time too.