As far as I can remember Multics didn't really have the concept of a
"mount point". All storage was single-level, i.e. segments (equivalent
in some respects to inodes, but they are also actually the value of the
segment register in the virtual memory hardware), and so files were
either physically in memory or paged out on physical disk devices or
similar, or even out on tape. Where they actually resided was entirely
and permanently hidden from the user. What was called the "filesystem"
was a form of database representing a hierarchical namespace which
pointed at all the known segments (files) regardless of where they were
actually stored.
Coming to it from a Unix perspective, it's like all storage (core, disk, tape) is mmap()ed.
The segment-name database then is just an index relating symbolic names to particular memory locations.
It all feels very upside-down to me, but that's probably because I grew up in Unix and never actually used a Multics system until I emulated one with dps8m.
Adam