Surely it's a strange decision to negotiate no gardening leave with RB if he was just going to retire anyway?

Maybe it was a just in case measure. So that he can say yes if the right opportunity arrives. Either way he is in a weird spot when it comes to designing a 2026 car. By the time he leaves Red Bull in 2025, the teams will have already laid down the fundamental design of their 2026 car, so he wouldn't be able to design it from the ground up. And in the era of the cost cap, the cars of the following years will likely be evolutions of the 2026 car rather than Newey's own design. We saw how long it took Mercedes to get away from their zero sidepod design and it's probably still hurting them.