While I'm not a lawyer, I'd guess that the points of contention might be...

with respect to services rendered for the sending State
Whether or not personal services are considered "services rendered for the sending state" might be open to some interpretation.

exempt from any obligations in regard to work permits imposed by the laws and regulations of the receiving State concerning the employment of foreign labor.
That would only cover obligations in regard to work permits, not obligations involving wages, working hours or conditions, etc.