From the passage, it seems that Frank Herbert did have writer's block and chose to write anyway.

???

"I have never had the problem of a writing block."

He means that what other people call "writing block" is just a label that bubbles up from within to describe not feeling it. Not feeling like writing. He prefers to label it, "I'd much rather go fishing." And with that belief, he writes anyway. Even expecting it to be rubbish. And then is surprised to look back and see it's comparable to those days on which he didn't have "writing block".

Well I can fairly believe other writers which have a nearly physical block and can't write a single word without trashing it immediately to the bin.