Welcome Dan Williams, new LibreOffice Developer focusing on UI/UX - The Document Foundation Blog
(blog.documentfoundation.org)
I'll give you an example since you clearly don't understand heuristics.
Look how OnlyOffice highlights selected buttons with a light gray tint.
LibreOffice on the other hand highlights them with very strong blue color, which draws the users attention and distracts them from the document.
There are many more very bad design choices that LibreOffice makes, but it's just a cluttered mess in general and can really put in some work to hide away all those buttons. Yes if you know where they are and use them every single day then it's more efficient, but it takes up a lot of (mind) space to see all those buttons all the time.