Wistron are a quiet little company that rarely release anything under their own brand. That's because they spend a lot of time designing and building for others. I'm not sure if Wistron are bigger or smaller than HTC, but most signs point to Wistron being one of the major contract manufacturers.
The agreement will cover Wistron's Android tablets, smartphones and ereaders. That last one is not just wishful thinking on the part of the marketing department; Wistronto get into that niche (plus they own Polymer Vision, a developer of flexible screen).
And I also know that Wistron have been making tablets; I recall at least 1 showing up last year at a trade show.