I think writer’s block is when you say to yourself, “I could write something, but it wouldn’t be good enough.” There’s no such thing as a complete inability to write a sentence.
He's right, of course: it's the fear of failure, of incompetence, or the fear of being perceived as incompetent, which stops so much useful work being done. If it's sub-standard, will it actually drive readers/customers away? John S Rhodes has an interesting take on the issue.
Best get on with it, then.