@Robert yes, your understanding is correct.
The problem with split histories is that in Pinegrow (as opposed to a normal code editor) certain operations change multiple pages and/or stylesheets. For example, creating a CSS rule with a class selector can also add the class to selected element(s). Undoing in one history, but not in others, would place the project in an inconsistent state that didn’t exist in the past. In addition, some operations also change the project-level database that was introduced with this release for Class Styles (and we have plans to use it for other things as well).
Undo system is one of the hardest thing to implement correctly. And having split histories complicates it even further, making it more difficult to fix bugs and improve it.