Stevenson's words are a bit of a puzzle, to be sure!
In the context, however, he is speaking of the relative merits of "doing nothing," or chillaxing, as the young people say nowadays, as opposed to working one's fingers to the bone and simply ending up with bony fingers! Having a little "me time" or some camaraderie from time to time is a good thing. You know, stop and smell the roses; get a fresh perspective; re-fuel for further exploits; and so on.
So if a person assiduously builds up a fortune--assuming the fortune was gained through judicious risk-taking in tandem with hard work, of course--and then loses it in a moment in time with an injudicious investment, of what benefit was all his assiduousness and risk-taking? Sounds kind of like the Old Testament book of Ecclesiastes: "Vanity of vanities; all is vanity!" In that book by King Solomon, the king asks more than once (and I paraphrase him very loosely), "What's the point in accumulating wealth when you could die any minute and your fortune go to strangers--or 'relatives' who crawl out of the woodwork for a piece of your estate? What's it all about? Is that all there is to life? Futility?"
As for scribblers, I guess Stevenson refers to the writers of editorials who are bound and determined to remove the mote in others' eyes but are oblivious to the log in their own. And then these scribblers wonder why people start avoiding them, or failing that, simply put up with the scribblers as just another "cross to bear."
So, if assiduous scribblings serve only to alienate people, how are Pharaoh, the Israelites, a pin, and a pyramid related to scribblings that alienate and hard work that ends in bankruptcy and premature death?
Well, let's see. Is there an analogy at work here? Perhaps. Could Pharaoh be the counterpart (analogically speaking) to the hard-work ethic? You know, "You can accomplish any momentous task if you only put your mind to it"? If so, then the hard workers are the counterpart to the Israelites, whom Pharaoh worked to death. But what of the pin?
Putting all the elements together, I suggest the hard worker who loses a fortune, the editorial writer who succeeds only in making enemies, and the Israeli slaves and the young man who work themselves into an early grave represent the "all work and no play" ethic. The editorial writer who thinks he's building a pyramid is in fact making only pins, and he succeeds in only alienating his friends. In other words, what to him is some great effort and worth every drop of sweat and blood he puts into it is not really worth the effort; it's all vanity, assuming he wants to continue having friends with whom he can kick back and enjoy a little gasconade and raconteurship--in other words, the fruits of idleness.
In essence, Stevenson goes to great lengths to say
"All work and no play makes Jack a dull boy."