I guess a design thinker is a designer that can make a concept, i.e. for an interface design, but doesn't need to provide the technical background to put the thoughts or the (written) concept into practice.
Platforms are usually meant to be different devices running different operating systems, such as Android devices, iWatch, iPhone, TV, Windows-PC, Mac, google glass, iPad, Linux-PC or whatever platform (a combination of hardware and software) is wanted for the project and usually described as 'cross platform'.
Channels are usually meant to be marketing channels. That could be a classical printed display (then the channel is a specific location and format), an advertising in some newspaper or magazine (then the channel is a certain magazine title, size, frequency, placement) or some TV channel (that channel is a name of a TV station, a specific broadcast time and/or frequency). More often a channel is used as a synonym for a certain web page or internet portal, specifically channel for digital advertising is mostly used for a social media ! platform ;-) like YouTube, Instagram, Facebook or anything else that can host digital advertising or whole campaigns, even a game or a real football match could provide several channels. It depends on the project and a little bit on your imagination, especially if you were responsible as a channel scout. So to say, almost everything could be a channel, if you can manage to use it in a reasonable fashion. Usually advertising has to be on a bunch of channels, what commonly is described as 'cross media'. Those different channels are mostly linked with each other, like a QR code on a display that takes you to some webpage that has some banner advertising that offers you some poll or contest. A channel can be imagined as a physical or virtual route from one person (e.g. a customer) to some other person (e.g. a salesman). As a designer that you're describing, you'll probably get hired as a creator or planner of such customer acquisition paths. Well, through cross media channels and over cross platforms.