One data point does not establish a lack of correlation.
Spelling ability is a poor overall indicator of intelligence and a poor indicator of overall communication skill.
The more effort your audience has to expend on mentally translating what you've written into English, the poorer a communicator you are. (I will agree that the inability to construct a coherent paragraph is a much greater impediment to communication.)
True but that was given as an example not a single data point. If you wish you can do a little research and see that spelling is considered a poor indicator of intelligence by virtually everyone that is in the business of measuring intelligence.
The more effort your audience has to expend on mentally translating what you've written into English, the poorer a communicator you are.
Typically in business communication via a written medium people can take advantage of spell check.