And Tim Sweeny made the offer to stop offering Epic exclusivity and even sell their games on Steam if Valve offered to provide their service to developers at the same rate as Epic.

Tim Sweeny didn't make an offer, he tried to make positive PR to EGS while trying to paint Valve as the bad guys; Valve obviously wouldn't charge the same rate as Epic because they include a lot more value for both user and developers than Epic does: to list a few of Valve services that Epic doesn't have:

Steam Workshop (hosting terabytes of content for absolutely free);
Family sharing;
Steam Link for game streaming;
Remote Play Together tech for all the major OSes;
Linux and Wine/Proton investments (which you could argue was an investment because of the Steam Deck, but that's an investment that benefits everyone, regardless of whether they own a Steam Deck or not);
Cloud save hosting;
Universal controller remapping interface compatible with all the major gamepads;

That's not to mention the benefits developers can get from Steam's platform and SDK:

Steam Input (for not needing to deal with custom implementations);
Steam Voice API (for in-game voice chats);
Steam Inventory and Trading Cards, which can result in extra cash for the developers;
Multiple networking options: Steam Game Servers, Steam Matchmaking & Lobbies, Steam Peer-to-peer Networking, etc.

If you ask me, I think Epic is the one charging way too much