[doodlesdev]

   > So the absence of a license means it defaults to exclusive copy right

Yes

   >  but can advertising it as open source be construed as a 'license'

I'm pretty sure the answer is no. There are no terms specified, no definition provided to what "open-source" is, and no information as to _what_ is licensed as open-source (i.e. the files, the compilation result, etc.).

General consensus with most licensing schemes is to add a license header to the top of every file, or otherwise specify that all files in a certain repository are subject to that license in a clear manner that everyone accessing these files will have access to (i.e. README file).

[burnished]

Yes, I probably should have omitted the example of the OP because I'm really more interested in the principle than the specific implementation.

My suspicion is that you could probably give verbal or informally written or offered license and that it would be valid from a legal perspective. I'm basing that off the existence of verbal contracts and how there is no process for licenses but instead a very practical consensus on the best way to communicate intent.

EDIT: In a sibling comment I verified this suspicion! Includes a couple links to short articles.