We are happy you enjoy our example. :)
Unfortunately, as Dave said, this effect can't be achieved in Storyline because it doesn't have functionality for background blur. Indeed, that would make a good feature request ;)
Given that the example needs working with the image editor, we have also created a blog post about how to recreate the glassmorphism effect in Figma. You can use Figma for free and it is quite a powerful browser-based design tool. You can read the step by step guide here:
https://fastercourse.com/how-to-create-glassmorphism-effect-in-figma/