The “but no simpler” part is the truism: you can’t make things simpler than is possible because that is impossible by definition. You could say “make it as simple as is prudent, but no simpler.” But that’s not so catchy!
Saying “make it as simple as possible” is still a helpful reminder, though, in the same vein as Rams’ tenth principle:
Good design is as little design as possible. Less, but better — because it concentrates on the essential aspects, and the products are not burdened with non-essentials.
Both suggest that simplicity is a virtue and that oversimplification is a danger, and that we must find an appropriate compromise. Perhaps Einstein’s maxim places a stronger emphasis on the risk of oversimplification. At any rate, both sayings can be difficult to apply.