I think it's just semantics, but what I found incorrect was saying that DHTML was just little snippets – that entirely depends on the use, I remember entire websites written with DHTML that were cutting edge at the time.
For me the reason to include ActiveX is because it came bundled with the web browser (IE) while for Flash you had to install the Macromedia plugin. Also, while Flash probably could interact with the DOM I personally almost never saw it done; I definitely remember ActiveX controls that would interface with the DOM, used almost in the way React and co are these days (!)
(I don’t feel ActiveX, applets or Flash were ever part of the meaning of DHTML. JavaScript/JScript/VBScript, absolutely. But these others had their own drawing areas and typically didn’t interact with the HTML DOM. They were just embedded alternate worlds, not DHTML.)