Apt removes packages and (with --purge option) their configuration files; i.e. global system files. If LibreOffice run at least once (or maybe even not run; I am not familiar with the install process of LO package), it created an user profile in the home directory, probably as ~/.config/libreoffice or so; and I am almost sure that the quoted “your user details in LibreOffice” belongs there.
So, at least as the first step, it is recommendable to remove ~/.config/libreoffice directory.
Disclaimer: I have a limited experience with XFCE desktop, so no idea what wizard from the bottommost pit of hell asked you to update these details; generally I can recommend to avoid all these wizards and prompts and simply manually edit configuration files, copy pictures etc – at least, you always know exactly what you are doing
EDIT: I did not refresh the page before posting, so sorry for partial dubbing the previous post.