Very misleading article. No - platforms do not mean app market places. App market places are platforms - but not the other way round. Author is just trying to improve his offline SEO. Not worth Hackernews stature.
What about Microsoft enterprise offerigs, oracle, salesforce? To me they also offer platforms, but they tend to mostly sell their own products on those.
Essentially, it is applicable to businesses that create cross-side network effects between developers and users. This includes Salesforce, but wouldn't include businesses that create developer tools or infrastructure for instance.