In IIS, Is it possible to execute a .HTML page as aspx?

Yes, it's possible. The link you provided is correct, although like you say it's for asp.dll instead of aspnet_isapi.dll.

The way to do it in IIS6 is:

• Edit the properties of the website
• Select the Home Directory tab
• Click the Configuration button
• Edit your .aspx entry and copy the path into your clipboard
• Create a new extension for .html (and .htm if you want) and paste what you have in the clipboard.

You'll also need to tell ASP.NET what to do with .html pages. To do this, in your web.config file, add a new entry to your httpHandlers section, like so:

<httpHandlers>
   <add path="*.html" verb="*" type="System.Web.UI.PageHandlerFactory" />
</httpHandlers>

In IIS7, if you are running your app pool in Classic mode then go to the website and the Handler Mappings section.
Take note of the entries for Path=.aspx.
Copy them and make them as .html.

Then do the same step to your web.config that I mentioned above.

If you're using integrated mode in IIS7, then you only need to do the web.config addition.