That is true - a felon cannot be prosecuted for violation of a gun registration law (in this case the NFA) because he would incriminate himself. But he can only incriminate himself because, as a convicted felon, his POSSESSION of the firearm is illegal. He cannot be prosecuted for failure to register the gun, but he can be prosecuted for POSSESSING it in the first place, which is why that decision really meant little in terms of jailing felons for gun possession. It meant only that the government brought the wrong charge.
Someone with no criminal record can be required to register his guns, since his possession of the gun is not illegal and he cannot incriminate himself.
The use of that decision to make registration proposals seem absurd is fine, but no judge would consider that it makes gun registration void or unconstitutional.
Jim