I'm not sure I understand it correctly, but I believe that there is a "basic" version that is free of charge, you only have to pay if you want some premium plugins and technical support.
Is this how the situation truly is?
yes.
Please see this link and this link for more information.
Anyone that wants to use TinyMCE in their commercial application or website are free to do so.
On top of this,the code is licensed under LGPL, so you may fork the code. Just remember to contribute your changes where other people can see it.(fork on github for example)
If you want support or want to contribute to the development, you are welcome to buy support or get addition plugins with the enterprise version. But only if you want.
EDIT: If you use a tool like Webpack, remember that the TinyMCE files should not be bundled and minified with the rest of your application. It should be a seperate Java Script and resource files. TinyMCE even has it's own CDN to help you with this.
"It should also be possible to exchange that dynamically linked library for an independently compiled build"