You’re correct. WP is just HTML and CSS under the skin. And yes. OptimizePress uses the built in editor. When using OP, you have a choice of using either the plugin or the theme. I’m using the theme because I’m using the OP blog settings. Do I have to do that? No. But I find it with the theme to be easier to give the site and the blog a consistent look and feel.
I’d already tried to bold some of the text with the editor and saw that it wasn’t bolded. I then looked at the HTML behind the text tab and saw the tags. When I saw it wasn’t working, I posted in the FB group and here. However, I didn’t know about the two plugins on the page you shared. I just tried CKEditor and it doesn’t work with OP. The editor doesn’t even open. That’s no surprise but OP has issues with a number of plugins. The good news is the RAD Text highlighter plugin works perfectly. Here’s the weird thing. The bolding is now working, too. 🙂 Thanks for the tip about those plugins.