Free software's not-so-eXZellent adventure

Three years.

And that immediately rules out all but largest companies, lone hobbists… and three-letter agencies.

They were hoping to get a reusable backdoor.

This is beyond what medium black-hat company may plan to do.

That's precisely the thing: exploits on black market command high price only if they are noncompromised.

Black market favors independent (even if easily detectable and patchable) exploits which you may sell to many buyers. Compromised, well-known exploits go down in price sharply.

This was an attempt to plant reusable exploit which was, presumably, was either planed to be used against one particular extra-high-profile target (which would still be there after three years!) or, alternatively, be reused again and again.

To reuse it again and again you need to “keep it in house” and ensure details wouldn't leak. To even have some high-profile target that is sitting there waiting for your attack you have to large organization which plans that encompass decades.

This, pretty much, rules out everyone but $THREE_LETTER_AGENCY es.

It's not about abilities to do such an exploit. It's about the use of such exploit. Only $THREE_LETTER_AGENCY (and ironically enough, lone hobbist) may benefit from something like this. And this looks like a work of a team which rules out “lone hobbist” hypnotises.