Individual civilian afghans are not embargoed by US law - it's not Iran. The Taliban are, of course, embargoed and listed in various things like the OFAC list.
Hundreds of thousands of ordinary Afghans have gmail accounts, many companies use google workspace or office365, etc. For example.
I think the point was that having an Afghani publish the source wouldn't really accomplish anything because Facebook could just go after whatever service was used to publish it, instead of the person who published it.
An Afghani can access GitHub or the chrome extension store, but those are both run by American companies who will obey Facebook's takedown requests.
At least that shifts the dangerous legal-financial burden onto google's lawyers, if they want to fight a takedown request to remove an extension from the extension store.
A takedown request is only binding on a well-capitalized firm like Google if it is based on some legal rationale that could survive a test in court. It isn't clear that the request under discussion has such a rationale. At the very least Google would take this to some court and force a judge to say something about it.
Once a firm grows accustomed to following orders from their competitors, bad times lie ahead.
Now you're into territory where yes, the extension is available, but nobody can find it and only the very technically astute will be able and willing to install it. FB achieves 99.9% of their goal.
Hundreds of thousands of ordinary Afghans have gmail accounts, many companies use google workspace or office365, etc. For example.