Reading it, think he's speaking conceptually. In practice it's more that PyCharm and WebStorm are both IDEA's base framework + language extensions. In the case of WebStorm you get IDEA + the HTML5/web dev extensions and in the case of PyCharm (commercial edition, anyway) you get IDEA + both the Python and the web dev extensions.
I believe RubyMine is the same, if not all of the other paid IDEs in the Toolbox. They all include WebStorm's functionality since you can build a web app in pretty much anything, at least on the back end.
It is not obvious from that Pycharm is actually a complete web developer IDE tool. But it has to be to support Django, it's html templating, and associated js.
Pycharm goes further and supports React and modern web toolchains as well. I suspect jetbrains would rather have you buy an intellij license if possible.
I believe RubyMine is the same, if not all of the other paid IDEs in the Toolbox. They all include WebStorm's functionality since you can build a web app in pretty much anything, at least on the back end.