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The problem here is the word "filesystem".  In the POSIX/Unix/Linux worlds, it is used to mean several different things.  
 1. The "filesystem" is sometimes the entire system of files, rooted at `/` and as presented to applications softwares by the operating system kernel.  With this meaning, people talk of POSIX operating systems having a "single *filesystem tree*", for example.  
 2. A "filesystem" is sometimes one (or more) *slice(s)* of a (or several) DASD(s) — one or more collections of contiguous disc sectors formatted as a single *volume* with a given format — as demarcated by some disc partitioning scheme.  With this meaning, people talk of, say, "formatting my *`/usr` filesystem*".  "`/usr`" here is the (expected) mountpoint of the volume, or (in some partitioning schemes) its identifying label.
 3. A "filesystem" is sometimes an abstract joinable tree of files and directories, presented by a filesystem driver (i.e. the VFS layer) to the rest of the system.  With this meaning, people talk of, say, "mounting the *proc filesystem* on `/proc`" or "using a *tmpfs filesystem* for `/tmp`".  

You're seeing meaning #1 when you see "`/usr/bin` filesystem".  It's a filesystem tree rooted at the directory `/usr/bin`.  Yes, Linux very much _does_ have the concept of directories.

Meaning #2 and meaning #3 are what you're mis-reading it as.  People are _not_ talking about `/usr/bin` as an on-disc data set, demarcated by disc slicing/partitioning.  Nor are they talking about `/usr/bin` as a particular type of filesystem driver, or a particular disc volume format.

# Further reading

* Ramesh Bangia (2010). "file system".  _Dictionary of Information Technology_. Laxmi Publications, Ltd.  ISBN 9789380298153.  p. 224.
* [File System](  "Base Definitions".  _The Open Group Base Specifications_.  Issue 7
IEEE 1003.1.  The Open Group.  2013.
* Binh Nguyen (2004-08-16). "filesystem".  [_The Linux Dictionary_]( Version 0.16.  p. 616.