I mean, no, because I think it's the 12% cut + the guaranteed money, because developers know even if they don't get the guaranteed money, having Epic will around will possibly lead to smaller cuts from Steam, in the long run.
It's literally just the guaranteed money. Devs were taking these deals to ensure a minimum of revenue against the risk of the game failing. The 12% cut means nothing because unless you (the dev) think that the guaranteed sales offer is lowballing your sales projections, you are never actually going to see the benefit of the 12% cut, you won't actually sell enough copies to exceed that initial sales guarantee and start seeing sales income.
This is especially true if you do eventually release the game on Steam in a year and you still haven't met those sales projections, because now the brunt of your sales are likely to come from Steam, and unless you're going to be running your own storefront to sell keys with 0% cut to Valve, you're accepting the 30% cut.
Valve isn't going to adjust their cuts due to the EGS. They have zero reason to do so.