By this time, I suppose you have already applied for an extension on your client's behalf.  You should take time to study the treaty for the aspects mentioned in more details.

There have been 5 protocols to the treaty and what you quoted is from an older one that has been superseded.  Also, re-sourcing pursuant to Article XXIV(6) will be necessary when preparing F.1116 for FTC, in order to alleviate double taxation on that particular type of income to the extent stated in the relevant articles.