> 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).