Larger themes like that tend to do things their own way instead of the regular WordPress way. What happens is they have excerpts in a lot of different places so they usually have a custom function for excerpts to give them flexibility in how the excerpts are handled in those different places. Not 100% sure about Divi, but that’s often the case with such themes.
Since it’s not a freely available theme I don’t have access to it so I’m not able to test it out and see what’s going on.
But it is a commercial theme and comes with premium support, so would be best to contact their support and ask about compatibility with Advanced Excerpt plugin, it is a free plugin so they have access to it and can test it.