It isn't - but considering that he is from a country where everybody speaks english, and that he is an international businessman I think you can expect a certain standard.
I'm a native English speaker and make mistakes writing my own language sometimes. I speak and write other languages and make more mistakes there.
Most people who are getting things done judge you by what you are getting done, not by whether you can spell the word clientele without looking it up on Google.
Fred Wilson has had spelling mistakes or simply used the wrong word in some instances on his blog - I don't think you can really use it as an accurate measure.
maybe english is not his first language?