The nuclear furnace smelting recipes seem to have the same in:out ratios and speeds as the normal furnaces, without the productivity bar. Is this by design?

The speed is the same yes but the ratio isn't. It's a 1.25 ratio since you get 5 resources for 4 inputs which since it's a recipe productivitiy means that with productivity 1.25 multiplies with 1.2 to make 1.5 which is a nice 2 to 3 ratio for steel.

That doesn't show up on a lot of recipe calculators in game. Is it doing something funny?

Are you going by output or input vs output? Cause my recipes definitely generate with 4->5 resources