It's not really quite as simple as that. It's a bit more nuanced and too time consuming to cover all of that here in detail but just to add...
You can still use Xerte Online Toolkits in Flash player mode either by changing the project properties, or when exporting, or via url parameters e.g. add &engine=flash or &engine=html to the end of an existing link or embed code
If you change the link to the Flash engine you get the same Flash interface that we used to have but obviously that won't play on devices like iPads without apps like Puffin Browser or without changing the URL to switch to html
The main difference between the developer tool and XOT now compared with what we used to have in much older versions is that there is no html 5 output or page types in the developer tool so whereas w used to keep the page types in both in sync that is no longer possible.
On the other hand if you don't need html 5 and have the Flash and actionscript skills then you can still create complete bespoke content with the developer too. However you won't really get any support with that and you won't be able to import those projects into XOT.
HTH
Ron