is module declaration mandatory?

[code-sur]

• My programming background is: Professional developer, 15 years working in industry, generalist.
• I want to learn Elm because: I like trying new languages, Brian Marick usually tweets about Elm and I've never used a fully functional language
• I was confused by:

The very first example of the guide does not compile: Introduction -> A Quick Sample
By just copy-paste the sample in a file (Main.elm) you get the following:

$ elm --version
0.19.1

$ elm make src/Main.elm 
Detected problems in 1 module.
-- MODULE NAME MISSING -------------------------------------------- src/Main.elm

I need the module name to be declared at the top of this file, like this:

    module Main exposing (..)

Try adding that as the first line of your file!

Note: It is best to replace (..) with an explicit list of types and functions
you want to expose. When you know a value is only used within this module, you
can refactor without worrying about uses elsewhere. Limiting exposed values can
also speed up compilation because I can skip a bunch of work if I see that the
exposed API has not changed.

It is not a big deal I know, one can just add the missing module line as the compiler suggests and it works but it didn't feel good having a compilation error in the first try of the hello world example

Later in Web Apps -> Modules it says:

Is that note valid? If it is, in which context? It doesn't seem to be valid with elm make or elm reactor

Thanks