i don't think so ... someone would've said something by now (that it's exclusive, i mean)
It might not even be moneyhats, but favourable incentives exclusively to certain developers. I dunno. I'm not a business person, I don't actually know what business development at Valve actually does if they're not finding ways to bend the rules to get developers on Steam. But everyone was probably sniffing around Destiny the second Activision dropped it, and you know Epic probably had some major cash to throw.
Maybe Bungie were burned by the BNet experience (it probably sold like shit on that client, on account of it not being a Blizzard game) and didn't want to repeat not being on Steam for a second time in a row, regardless of the cash? Or maybe Epic just didn't give them enough money to make it work? Or maybe Valve just made them the better offer?
With the way Valve operates, I'm not sure a moneyhat or favourable incentive (revenue share, maybe?) is something they'd necessarily gloat about. It'd just be a thing they did to get a thing, and they don't need to sing it from the rooftops because the fact that Destiny 2 is on Steam and nowhere else will speak for itself. Plus it's probably a pandora's box they really don't want many to know they've opened, for the obvious reason that everyone will then come to them cap in hand for some of those sweet incentives or straight up cash.
Plus I really don't think Valve are sitting on their arses doing nothing about EGS. They can't rely on blind faith that the market will sort this out, because Epic has clearly found a flaw in the free market (not making it free, essentially) and are exploiting it with every dime they have. They must be having conversations with developers thinking of going to Epic, or developers that have gone to Epic (if only to ask why, and what they could do better), and there's also a lot of pent up frustration on their own channels (and probably Gabe's inbox) about what Valve is actually doing to stop this poaching going on. I saw in one Steam forum a bunch of people were demanding publishers who use Steam as free advertising before doing a switcheroo to Epic get blacklisted by Valve. Not necessarily something I support, but it's clear that customers are getting more and more agitated about this.
Of course this is all conjecture on my part, but I just don't believe for a second they aren't doing a thing to try and curb this.