So... I have a wild theory.
Things that have been pointed out about Hegemony tech are that it has a crazy long design lifetime, and a lot of limits built in to it to prevent it being used in unapproved ways. Together with the natural lack of curiosity that most of the Hegemony races seem to exhibit, I think it looks like they've been set up in technological stasis, similar to what Langhorne pulled on Safehold. The ridiculous time-to-failure means that you never build up a cottage industry of people who know how to keep your old busted aircar running because you don't get very many old busted ones. And you don't get people working on improving the tech because it's designed not to be user-serviceable (it does what it says on the tin, no more, no less) and anyway, it works well enough that the Hegemony races say it ain't broke and don't see a reason to try to improve it.
If we accept a tech stasis, who did it? The mysterious Fifth Race are obvious culprits. But what does it achieve? The obvious thing it does is keep Hegemony ship speeds, and hence communication and expansion speeds, down. That means that the Shongairi get to Earth when they do, not a few years earlier when we'd probably not have been able to push them hard enough to kick Vlad out of his non-interference mode.
So, what if one of the things mucking round with the artificial gravity lets you do, apart from make super-speedy hyperdrives, is
time travel. Say the rules of it are the "stable time loop" variety, so we have to let the Shongairi invasion happen so that we get the tech to time travel. But it has to happen when it does, which means that the Hegemony can't get here too early. So we go back and help found the Hegemony. We "helpfully" provide them with better hyperdrives and gravity generators and what-not than they have at the time, but carrying this sneaky sting in the tail. Then we do something to scare the rest of the races (possibly just be omnivorous and aggressive) and "lose to their superior numbers" in a war of extermination which then gets quietly written out of history. That writing-out-of-history also conveniently means that no living Hegemony people recognise us when they see Agincourt. So what we did was set up the Shongairi to turn up and hit us, but at a time when we could - just - take it.
A stable time loop also makes sense of Vlad. One supersoldier is an awful risk. What if he'd been in a big city when the Shongairi bombarded it? Could his nanotech really stand up to an orbital strike to the face? But if we're also The Other Guys then we have foreknowledge that he won't be under a kinetic strike, and also that he'll (eventually) use his powers responsibly.
So anyway, that's my wild speculation. We aren't now the Fifth Race or the Other Guys, but we were/will be/whatever.
There is one more little thing which occurred to me as I was writing this which is definitely in nutty secret handshake territory. I take it others noticed Janet Frasier of Stargate SG1 making a cameo appearance in the Honorverse as Honor's doctor when she and Hamish have their little accident? So Weber watched the series. And the Asgard in SG1 refer to humans as
the Fifth Race...