When we started flatpak’ing LibreOffice, we made builds of the LibreOffice “Fresh” stream available on the Document Foundation download pages. Since LibreOffice 5.4.2, we now provide such builds through Flathub.
The benefit to you as a user is that there are builds for more architectures (currently aarch64, i386, and x86_64), and that you need to keep track of one less single-app Flatpak repo. (Check out what else is available in the Flathub repo!)
The benefit to me as provider of the original Document Foundation flatpak builds is that the Flathub infrastructure now does those builds for me. We will stop providing the Document Foundation builds after the current LibreOffice 5.4.3.