You absolutely can use LearnDash and Divi together without Divi LearnDash Kit. But using Divi LearnDash Kit streamlines the process of adding, configuring and styling LearnDash components into Divi Builder posts / pages.
Using LearnDash without Divi LearnDash Kit
If you opt to use LearnDash on Divi without Divi LearnDash Kit then, if you want to use LearnDash components within a Divi Builder post / page, you will need to use LearnDash shortcodes which you can place within Divi Text modules or Code modules, e.g.:
Using LearnDash with Divi LearnDash Kit
The main benefit of Divi LearnDash Kit is the addition of new Divi Modules that correspond to the LearnDash shortcodes, streamlining the process of adding LearnDash components into Divi Builder layouts.
For example the shortcode / code module combination above can be replaced with the "LearnDash Course List" module, like so:
Now the component can be configured using the module options, rather than shortcode attributes.
The module can also be styled using the built-in style options, rather than requiring custom CSS. There are also some additional options added to some of the modules which would otherwise require custom code to implement.
The final function of Divi LearnDash Kit is to smooth out some incompatibilities between Divi and LearnDash. They wouldn't stop you from using Divi and LearnDash without Divi LearnDash Kit, but are minor annoyances that can be avoided that might otherwise require a bit of custom code to resolve.
Whether you opt to use Divi and LearnDash alone, or enhance them with Divi LearnDash Kit, feel free to ask any questions you have around getting Divi and LearnDash to work together. I'll be happy to help.