I just noticed we can get back to the main site by clicking the LingQ icon in the top right near the search bar instead of the icon in the top left. It does look like you need to be completely scrolled to the top of the page though, otherwise the header is replaced with the title of the thread you’re currently reading.
EDIT: Looks like they added the normal menu buttons to the top right to get back to the main site (Lessons, Tutors, etc.)