The lids on my NOVA Processors do very much the same thing but I follow a strict procedure when I have finished printing to make sure they are optimised.

I top up/replenish the developer, stop bath, and blix when I have finished, wipe the edges of the tank dry and then with the floating lids in place, cover the whole of the top of the tank with cling film to make it as airtight as possible. When the solutions cool down, if I have obtained a good seal, the cling film allows a semi-vacuum to form in the tank. (I only worry about the developer) You can tell this has happened when the surface of the cling film which was previously flat, takes on a concave depression.

I have done this for as long as I have been processing colour and the RA4 developer (Kodak Ektacolour) has lasted as long as 18 months so long as it replenished properly. I only need to change it when the build up of tar gets too much.

So yes, in the right circumstances they do work.